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?).

Start by reviewing how setup.py --classifiers is invoked and how setup.py metadata is currently read. Compare the minimal setup.py with the referenced setup.cfg, focusing on python_requires and the separate request to modify metadata. Done means setup.cfg metadata is read correctly, with modification support treated as a separately scoped decision.

Written by the indexing model from the issue text.