cannot install airflow git package from master branch


### Issue description
Trying to install apache-airflow from master branch but getting this error 
when I run `pipenv install`

```
 pipenv install 
Courtesy Notice: Pipenv found itself running within a virtual environment, so it will automatically use that environment, instead of creating its own for any project. You can set PIPENV_IGNORE_VIRTUALENVS=1 to force pipenv to ignore that environment and create its own instead. You can set PIPENV_VERBOSITY=-1 to suppress this warning.
Pipfile.lock not found, creating…
Locking [dev-packages] dependencies…
Locking [packages] dependencies…
Building requirements...
Resolving dependencies...
✘ Locking Failed! 
Traceback (most recent call last):
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 807, in <module>
    main()
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 803, in main
    parsed.requirements_dir, parsed.packages, parse_only=parsed.parse_only)
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 785, in _main
    resolve_packages(pre, clear, verbose, system, write, requirements_dir, packages)
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 753, in resolve_packages
    requirements_dir=requirements_dir,
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 736, in resolve
    req_dir=requirements_dir
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 1386, in resolve_deps
    req_dir=req_dir,
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 1091, in actually_resolve_deps
    deps, index_lookup, markers_lookup, project, sources, req_dir, clear, pre
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 649, in create
    pre=pre, clear=clear
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 463, in get_metadata
    req, resolver=transient_resolver
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 540, in get_deps_from_req
    req_list, lockfile = get_vcs_deps(reqs=[req])
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 1874, in get_vcs_deps
    with temp_path(), locked_repository(requirement) as repo:
  File "/usr/local/opt/python/Frameworks/Python.framework/Versions/3.7/lib/python3.7/contextlib.py", line 112, in __enter__
    return next(self.gen)
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 2030, in locked_repository
    with requirement.req.locked_vcs_repo(src_dir=src_dir) as repo:
  File "/usr/local/opt/python/Frameworks/Python.framework/Versions/3.7/lib/python3.7/contextlib.py", line 112, in __enter__
    return next(self.gen)
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/requirements.py", line 2212, in locked_vcs_repo
    self._parsed_line.vcsrepo = vcsrepo
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/requirements.py", line 916, in vcsrepo
    kwargs=wheel_kwargs,
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/setup_info.py", line 1937, in create
    created.get_initial_info()
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/setup_info.py", line 1766, in get_initial_info
    self.update_from_dict(self.parse_setup_py())
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/setup_info.py", line 1479, in update_from_dict
    for section in set(list(extras_require.keys())) - set(list(self.extras.keys())):
AttributeError: 'str' object has no attribute 'keys'

```
My Pipfile is as follows 
```
[[source]]

url = "https://pypi.python.org/simple"
verify_ssl = true
name = "pypi"


[packages]

apache-airflow = {git = "https://github.com/apache/airflow.git", editable = true, ref = "master", extras = ["slack", "postgres", "redis", "crypto", "s3", "celery"]}


[dev-packages]



[requires]

python_version = "3.7"

[pipenv]
allow_prereleases = true
```

### Expected result

pipenv install should work 

### Actual result
```
$ pipenv install --verbose
Courtesy Notice: Pipenv found itself running within a virtual environment, so it will automatically use that environment, instead of creating its own for any project. You can set PIPENV_IGNORE_VIRTUALENVS=1 to force pipenv to ignore that environment and create its own instead. You can set PIPENV_VERBOSITY=-1 to suppress this warning.
Pipfile.lock not found, creating…
Locking [dev-packages] dependencies…
Locking [packages] dependencies…
Building requirements...
Resolving dependencies...
⠇ Locking...Traceback (most recent call last):
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 807, in <module>
✘ Locking Failed! 
    main()
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 803, in main
    parsed.requirements_dir, parsed.packages, parse_only=parsed.parse_only)
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 785, in _main
    resolve_packages(pre, clear, verbose, system, write, requirements_dir, packages)
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 753, in resolve_packages
    requirements_dir=requirements_dir,
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 736, in resolve
    req_dir=requirements_dir
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 1386, in resolve_deps
    req_dir=req_dir,
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 1091, in actually_resolve_deps
    deps, index_lookup, markers_lookup, project, sources, req_dir, clear, pre
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 649, in create
    pre=pre, clear=clear
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 463, in get_metadata
    req, resolver=transient_resolver
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 540, in get_deps_from_req
    req_list, lockfile = get_vcs_deps(reqs=[req])
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 1874, in get_vcs_deps
    with temp_path(), locked_repository(requirement) as repo:
  File "/usr/local/opt/python/Frameworks/Python.framework/Versions/3.7/lib/python3.7/contextlib.py", line 112, in __enter__
    return next(self.gen)
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 2030, in locked_repository
    with requirement.req.locked_vcs_repo(src_dir=src_dir) as repo:
  File "/usr/local/opt/python/Frameworks/Python.framework/Versions/3.7/lib/python3.7/contextlib.py", line 112, in __enter__
    return next(self.gen)
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/requirements.py", line 2212, in locked_vcs_repo
    self._parsed_line.vcsrepo = vcsrepo
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/requirements.py", line 916, in vcsrepo
    kwargs=wheel_kwargs,
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/setup_info.py", line 1937, in create
    created.get_initial_info()
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/setup_info.py", line 1766, in get_initial_info
    self.update_from_dict(self.parse_setup_py())
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/setup_info.py", line 1479, in update_from_dict
    for section in set(list(extras_require.keys())) - set(list(self.extras.keys())):
AttributeError: 'str' object has no attribute 'keys'
Traceback (most recent call last):
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 807, in <module>
    main()
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 803, in main
    parsed.requirements_dir, parsed.packages, parse_only=parsed.parse_only)
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 785, in _main
    resolve_packages(pre, clear, verbose, system, write, requirements_dir, packages)
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 753, in resolve_packages
    requirements_dir=requirements_dir,
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/resolver.py", line 736, in resolve
    req_dir=requirements_dir
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 1386, in resolve_deps
    req_dir=req_dir,
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 1091, in actually_resolve_deps
    deps, index_lookup, markers_lookup, project, sources, req_dir, clear, pre
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 649, in create
    pre=pre, clear=clear
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 463, in get_metadata
    req, resolver=transient_resolver
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 540, in get_deps_from_req
    req_list, lockfile = get_vcs_deps(reqs=[req])
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 1874, in get_vcs_deps
    with temp_path(), locked_repository(requirement) as repo:
  File "/usr/local/opt/python/Frameworks/Python.framework/Versions/3.7/lib/python3.7/contextlib.py", line 112, in __enter__
    return next(self.gen)
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/utils.py", line 2030, in locked_repository
    with requirement.req.locked_vcs_repo(src_dir=src_dir) as repo:
  File "/usr/local/opt/python/Frameworks/Python.framework/Versions/3.7/lib/python3.7/contextlib.py", line 112, in __enter__
    return next(self.gen)
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/requirements.py", line 2212, in locked_vcs_repo
    self._parsed_line.vcsrepo = vcsrepo
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/requirements.py", line 916, in vcsrepo
    kwargs=wheel_kwargs,
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/setup_info.py", line 1937, in create
    created.get_initial_info()
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/setup_info.py", line 1766, in get_initial_info
    self.update_from_dict(self.parse_setup_py())
  File "/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv/vendor/requirementslib/models/setup_info.py", line 1479, in update_from_dict
    for section in set(list(extras_require.keys())) - set(list(self.extras.keys())):
AttributeError: 'str' object has no attribute 'keys'
```

### Steps to replicate

attached above


<details><summary>$ pipenv --support</summary>

Pipenv version: `'2020.6.2'`

Pipenv location: `'/usr/local/Cellar/pipenv/2020.6.2/libexec/lib/python3.8/site-packages/pipenv'`

Python location: `'/usr/local/Cellar/pipenv/2020.6.2/libexec/bin/python3.8'`

Python installations found:

  - `3.7.5`: `/usr/local/bin/python3`
  - `3.7.5`: `/usr/local/bin/python3.7m`
  - `3.7.5`: `/usr/local/bin/python3.7`
  - `3.7.3`: `/usr/bin/python3`
  - `2.7.16`: `/usr/bin/python2`
  - `2.7.16`: `/usr/bin/python2.7`
  - `2.7.15`: `/usr/local/bin/python2`
  - `2.7.15`: `/usr/local/bin/python2.7`

PEP 508 Information:

```
{'implementation_name': 'cpython',
 'implementation_version': '3.8.3',
 'os_name': 'posix',
 'platform_machine': 'x86_64',
 'platform_python_implementation': 'CPython',
 'platform_release': '19.5.0',
 'platform_system': 'Darwin',
 'platform_version': 'Darwin Kernel Version 19.5.0: Thu Apr 30 18:25:59 PDT '
                     '2020; root:xnu-6153.121.1~7/RELEASE_X86_64',
 'python_full_version': '3.8.3',
 'python_version': '3.8',
 'sys_platform': 'darwin'}
```

System environment variables:

  - `PATH`
  - `TERM_PROGRAM`
  - `TERM`
  - `SHELL`
  - `TMPDIR`
  - `TERM_PROGRAM_VERSION`
  - `TERM_SESSION_ID`
  - `ZSH`
  - `USER`
  - `SSH_AUTH_SOCK`
  - `PAGER`
  - `LSCOLORS`
  - `_`
  - `LaunchInstanceID`
  - `PWD`
  - `LANG`
  - `XPC_FLAGS`
  - `XPC_SERVICE_NAME`
  - `HOME`
  - `SHLVL`
  - `LESS`
  - `LOGNAME`
  - `SECURITYSESSIONID`
  - `__CF_USER_TEXT_ENCODING`
  - `PIP_DISABLE_PIP_VERSION_CHECK`
  - `PYTHONDONTWRITEBYTECODE`
  - `PIP_SHIMS_BASE_MODULE`
  - `PIP_PYTHON_PATH`
  - `PYTHONFINDER_IGNORE_UNSUPPORTED`

Pipenv–specific environment variables:


Debug–specific environment variables:

  - `PATH`: `/usr/local/Cellar/pipenv/2020.6.2/libexec/tools:/usr/local/opt/terraform@0.11/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/go/bin:/usr/local/MacGPG2/bin:/Library/Apple/usr/bin:/Applications/Wireshark.app/Contents/MacOS`
  - `SHELL`: `/bin/zsh`
  - `LANG`: `en_US.UTF-8`
  - `PWD`: `/Users/nchakrapani/`


---------------------------

Contents of `Pipfile` :

```toml
[[source]]

url = "https://pypi.python.org/simple"
verify_ssl = true
name = "pypi"


[packages]

apache-airflow = {git = "https://github.com/apache/airflow.git", editable = true, ref = "master", extras = ["slack", "postgres", "redis", "crypto", "s3", "celery"]}


[dev-packages]



[requires]

python_version = "3.7"

[pipenv]
allow_prereleases = true

```

</details>


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cannot install airflow git package from master branch #4342

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