feat: pyproject-metadata

[henryiii]

Start of adding the bulk of pyproject-metadata. Prerequisites:

• PEP 639: Implement License-Expression and License-File #828 (includes PEP 639) - Note, I'll probably add the (I believe) missing validation errors for packaging.metadata after that is merged.
• feat: support writing metadata #846 (builds off that PR, which itself is waiting for the PEP 639 one)
• chore: speed up mypy a little #836 (needs the MyPy bump)

TODO:

• Add tests ported from pyproject-metadata
• Rename things for consistency (current naming is a mess)
• Ensure error style is consistent with other parts of packaging
• Ensure packaging.metadata throws all errors in .validate_metadata, and then remove .validate_metadata
• Add docs
• Ensure import style is consistent