This code does a simple copy of keywords from setuptools. I've made it
so that if the list of keywords is empty, no entry will be added to
pyproject.toml, mostly for consistency with how we handle other fields.

These tests cover the simple cases and some edge cases. I added them to
an existing test file because there wasn't enough there to make it
important to split these tests into their own file.

This commit adds the expected result for the keywords field to
test_everything().

I changed one thing in the input setup.cfg file: the list of keywords
was space-separated, but it should have been comma-separated. In fact
the core metadata specification was changed from using spaces to commas
precisely because that's what distutils and setuptools did.
  https://packaging.python.org/en/latest/specifications/core-metadata/#core-metadata-keywords