PEP 639: Implement License-Expression and License-File

[ewdurbin]

~Work in progress, current open questions:~

• [x] ~Is the dependency on license-expression OK? Should vendoring or a new parser be considered?~
  • ~went the vendoring route per feedback from @abravalheri~
  • migrated to a pure python parser based on pypa/hatchling's implementation
• [x] ~How much validation to apply to License-File, as-is as far as metadata goes the only clear spec is "Path delimiters MUST be the forward slash character (/), and parent directory indicators (..) MUST NOT be used.". It is unclear how to check for other delimiters, as paths may have no delimiters whatsoever~
  • validations with path lib seem to fit the bill!
• [x] ~How to handle deprecations/conflicts. License and License-Expression are mutually exclusive, but it doesn't appear that such a conflict has existed before.~
  • Based on the current behavior of the library, this kind of concern appears to be left to the caller. The metadata module appears to be focused on correctly parsing individual fields.