Warn when python_version markers aren't valid PEP 440 versions

[gyanu2507]

python_version >= "3.9." isn't a PEP 440 version, so the comparison currently falls back to string compare with no indication (and "3.10" >= "3.9." is False).

Warn for < <= == != >= > on python_version / python_full_version when that happens. in / not in and platform_release are unchanged.

Fixes #633

[gyanu2507]

The Downstream twine failure is also on main and isn't related to this change.

Both runs fail on the same single test with the same assertion:

FAILED tests/test_check.py::test_fails_rst_no_content - AssertionError: assert False
1 failed, 227 passed, 1 deselected

That's twine's own README-rendering check, which doesn't go through marker evaluation. main at b9d249f (perf(tests): disable more unused pytest ...) has it as its only failing check; 053c884 and earlier were green, so it started upstream of this branch.

The other 61 checks here pass.

[henryiii]

Thanks! Fixing the CI in #1400.