Resolve package build warnings

[kurtmckee]

When building pyjwt, a number of warnings are thrown:

SetuptoolsDeprecationWarning: License classifiers are deprecated.

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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.
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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 2026-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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These warnings are addressed in the obvious ways,
and the build system (setuptools) now requires 77.0.3 or higher
to ensure base compatibility with the changes.