use spdx license metadata in pyproject.toml

[ThomasWaldmann]

Updated setuptools minimum version with spdx support in build-system requirements.

This gets rid of this warning:

  Running command Building editable for moin (pyproject.toml)
  /private/var/folders/61/qtqwzmxd7t1fb19zck5k3q0h0000gn/T/pip-build-env-bohc36n9/overlay/lib/python3.13/site-packages/setuptools/config/_apply_pyprojecttoml.py:82: SetuptoolsDeprecationWarning: `project.license` as a TOML table is deprecated
  !!

          ********************************************************************************
          Please use a simple string containing a SPDX expression for `project.license`. You can also use `project.license-files`. (Both options available on setuptools>=77.0.0).

          By 2026-Feb-18, you need to update your project and remove deprecated calls
          or your builds will no longer be supported.

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

  !!
    corresp(dist, value, root_dir)
  /private/var/folders/61/qtqwzmxd7t1fb19zck5k3q0h0000gn/T/pip-build-env-bohc36n9/overlay/lib/python3.13/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 (GPL)

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

  !!
    dist._finalize_license_expression()
  /private/var/folders/61/qtqwzmxd7t1fb19zck5k3q0h0000gn/T/pip-build-env-bohc36n9/overlay/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 (GPL)

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

  !!
    self._finalize_license_expression()