add specific supported python versions to classifiers metadata

[fmigneault]

I would like to propose this simple metadata update.

Multiple tools like https://pyup.io, https://requires.io, etc. parse this package metadata to indicate whether they support specific versions and environments. Because werkzeug does not indicate them explicitly, they are flagged as incompatible for Python 3, although the docs and other configurations report that python_requires = >= 3.7.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[davidism]

Those tools should check python_requires instead (or additionally). It specifies that all versions >= 3.7 are supported (as long as that version of Werkzeug is also supported). Maintaining classifiers doesn't work because it requires a new release of Werkzeug to indicate "support" for a new release of Python, even if no changes are required for that support.

Please open feature requests to support requires-python metadata on those tools instead.