Support license-files [PEP-639]

[sjlongland]

So, in issue #32 I was thrown off-course by the setuptools documentation, which documented both license and license_files fields.

It was not clear whether the license field was sometimes read from a file instead of being verbatim text, or how the license_files influenced this field.

Experimental work suggested the two fields do not interact; a license file described in license_files does not yield text appearing in the license field. Furthermore, setuptools does not support the file: prefix to denote text as coming from an external file as it does with other fields.

Having seen the library mentioned in issue #74 and reading the official Python package specification linked within (https://packaging.python.org/en/latest/specifications/core-metadata/#license), it is apparent this is a setuptools extension, undergoing discussion for inclusion under PEP-639 (https://peps.python.org/pep-0639/).

As such, we're probably better off waiting until a decision is made on PEP-639 before implementing it.

[diazona]

Yep, definitely makes sense to me to stick with the PEPs that have officially been accepted.

I suppose we could support 639 as an option, but that's not worth doing for our initial release IMO.

[diazona]

Looks like it's happening: https://floss.social/@hroncok/113044747383279536