Add PEP 639 license metadata

[diazona]

With the acceptance of PEP 639 (some time ago), the License metadata field is deprecated in favor of License-Expression and License-File. We should update the setuptools configuration to make it generate those new fields. Otherwise we get errors when building the package such as the following:

/tmp/build-env-7ryum683/lib/python3.12/site-packages/setuptools/dist.py:759: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

        ********************************************************************************
        Please consider removing the following classifiers in favor of a SPDX license expression:

        License :: OSI Approved :: MIT License

        See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
        ********************************************************************************

!!
  self._finalize_license_expression()

[diazona]

NB switching away from setuptools is also on the table. (Personally I like hatchling.) But it would probably make sense to finish #126 first.