PyPI: Python version from classifiers vs. requires_python

In #5550 I noted that to get pyversion, you need to use classifiers. However, there are a lot of python libraries out there that also use [`python_requires`](https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires) instead.
Instead of making a feature proposition there, I decided to make a new issue that was directed at this task instead of using an existing 'question' issue.

I'm proposing a feature where the pyversions look for python_requires before parsing classifiers.

Find the pyversion implementation and the logic that parses Python classifiers; the issue does not name specific files or tests. Check how python_requires is represented in PyPI metadata, then verify that it is considered before classifiers and add or update coverage for the resulting version selection.

Written by the indexing model from the issue text.