Use PEP 639 license metadata

[brianschubert]

c.f. mypyc/ast_serialize#43, python/mypy#18821

I verified that the metadata in the built wheels is updated as expected:

$ uv build && unzip -p dist/librt-0.8.1-py3-none-any.whl librt-0.8.1.dist-info/METADATA | grep -i license
...
License-Expression: MIT
License-File: LICENSE
Dynamic: license-file

[brianschubert]

(note: librt already uses setuptools v77+, which has support for PEP 639)