[FR] Implement PEP 639

[befeleme]

See: https://peps.python.org/pep-0639

Additional context:

• The full implementation, including the Core Metadata bump to 2.4, will require also the support on the side of PyPI first: Implement PEP 639 pypi/warehouse#16620

• There's a plan to add validation data and parser to packaging: Add SPDX license data pypa/packaging#799 + PEP 639: Implement License-Expression and License-File pypa/packaging#828