Implement PEP 639

[befeleme]

What's the problem this feature will solve?
Standardized licensing metadata will in time unify the currently largely chaotic landscape.

Describe the solution you'd like
See: https://peps.python.org/pep-0639
PEP 639 has been provisionally accepted with one of the conditions being implementation in PyPI.

[ewdurbin]

PyPI uses https://github.com/pypa/packaging to parse and validate metadata, so it will need an update to support the new metadata version, keys, and deprecations.

I've opened draft PRs for that and the packaging docs at:

• PEP 639: Implement License-Expression and License-File pypa/packaging#828
• PEP 639: Add documentation for Metadata 2.4, License-Expression and License-Field pypa/packaging.python.org#1595

Once those PRs (or something that supersedes them) are merged, implementation in PyPI can begin.