SPDX license expression

[Chesyon]

Silences the following warning when building:

      /tmp/pip-build-env-7327f090/overlay/lib/python3.12/site-packages/setuptools/config/_apply_pyprojecttoml.py:61: 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 or later (GPLv3+)
      
              See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
              ********************************************************************************
      
      !!

[Frostbyte0x70]

Thanks!