[Feature request] Implement PEP 639

[befeleme]

PEP 639 has been provisionally accepted with one of the conditions being implementation in two build backends.
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 packaging#799

[cdce8p]

I've opened PRs to add support for both License-File and License-Expression, see #705 and #707. The final support for version 2.4 is still dependent on upstream support. AFAIK twine still doesn't support it.