pyproject.toml: PEP 639 compatibility

[adamjstewart]

Reverts #199
Fixes #200

PEP 621 is superseded by PEP 639. The previous syntax is deprecated and will be removed in a future version of setuptools.

@Bluefissure @EpicWink @mattip

[EpicWink]

You may want to add license-files = ["LICENSE"] to keep the license explicitly referenced (setuptools still includes the license file in the sdist and wheels anyway)

[adamjstewart]

The default value of license-files seems to include this. We can make it more explicit if we want.