When testing WritePyproject, we need to give it a Distribution object,
which can be created either by calling setuptools.setup() or by manually
calling the constructor and passing in a dict of attributes. The latter
way is faster to run because it doesn't have to create and read a file,
but it's a little unwieldy to write that code over and over again. Plus,
we've had problems where the Distribution object will read
setuptools configuration files in whatever is the current working
directory when the test runs, and will set some of its attributes based
on that configuration. (Specifically, it seems to use setuptools_scm to
set the version number, but maybe this would apply to other dynamic
attributes, I'm not sure.)

This commit adds a fixture, make_write_pyproject, that will address
the problem in two ways:

- It ensures that any test requesting the fixture will run in an empty
  working directory
- It also provides a convenience function that creates a Distribution
  from the given keyword arguments and then creates a WritePyproject
  from that Distribution.

We can now use this fixture to avoid having to run the full setuptools
configuration procedure if we don't have to.

In this commit, I'm changing the make_write_pyproject fixture to set
a default name and version if those are not passed as keyword arguments.
Those are the only two attributes required by the Distribution
constructor, but we often don't care what their values are. If a test
does care, the values are accessible as class attributes DEFAULT_NAME
and DEFAULT_VERSION.

As a side effect of making the fixture return a class, I had to drop
typing information, because names in conftest.py aren't made available
to test code except through fixtures. It should be possible to bring
back the typing information by creating an extra package in the main
project for test helper code, or by using something like
pytest-helpers-namespace, but I don't think that's strictly necessary
to get this code merged.

This commit changes all the tests that used to manually create
Distribution objects to use the new make_write_pyproject fixture.