Release v4.0.21

Use SPDX license expression in setup.py

Fixes a build-time warning from setuptools:

/tmp/build-env-7cm02e4g/lib/python3.12/site-packages/setuptools/dist.py:765: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

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

        License :: OSI Approved :: GNU General Public License v3 (GPLv3)

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

!!
  self._finalize_license_expression()