Improve license metadata for PEP 639

[mwtoews]

This improves the license metadata to use PEP 639 which requires setuptools 77 or later. It resolves a build message:

SetuptoolsDeprecationWarning: `project.license` as a TOML table is deprecated
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        Please use a simple string containing a SPDX expression for `project.license`. You can also use `project.license-files`. (Both options available on setuptools>=77.0.0).

        By 2027-Feb-18, you need to update your project and remove deprecated calls
        or your builds will no longer be supported.

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