Support reading metadata from setup.cfg

https://gitlab.com/anarcat/undertime is an example of a package that has a minimal setup.py and all the metadata is stored in setup.cfg.

check-python-versions is able to get classifiers from this package (by running setup.py --classifiers), but it doesn't notice python_requires.  Also, it's unable to modify anything.  It would be nice if it could (but maybe that's a separate wishlist bug?).