Python package: use a SPDX license expression

Description

When building a Python package on sid, the following deprecation notice is displayed (example).

* Getting build dependencies for wheel...
/tmp/build-env-1jrlqq2g/lib/python3.13/site-packages/setuptools/dist.py:759: 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.
        ********************************************************************************
!!
  self._finalize_license_expression()

Tasks

• Switch setup.cfg to use a SPDX license expression instead of License classifiers (details).

Time estimation

• Complexity: negligible (0.1 day)
• Uncertainty: low (x1.1)
• Reference (adapted)