pep639: setuptools license and license-files fields

[bjlittle]

🚀 Pull Request

Description

This pull-request mops up some tech debt to comply with PEP639 for setuptools, also see license and license-files.

Additionally avoids the following setuptools deprecation warning:

!!
          ********************************************************************************
          Please consider removing the following classifiers in favor of a SPDX license expression:
          License :: OSI Approved :: BSD License
          See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
          ********************************************************************************
!!

See Deprecated license classifiers.

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