License classifiers are deprecated since setuptools 77

[Jun Omae]

cf.

• ​https://setuptools.pypa.io/en/stable/history.html#v77-0-0
• ​pep:639#deprecate-license-classifiers
• ​pep:639#deprecate-license-field

$ python -Wdefault setup.py egg_info
/venv/lib/python3.10/site-packages/setuptools/dist.py:760: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

        ********************************************************************************
        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.
        ********************************************************************************

!!
  self._finalize_license_expression()