The setup.py file has been recently updated to specify the license as license='GPL-3.0-or-later'.

Anyway in the classifiers section it is is specified:

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

which corresponds to GPL-3.0-only, while GPL-3.0-or-later corresponds to

License :: OSI Approved :: GNU Lesser General Public License v3 or later (LGPLv3+)

Please clarify the licensing.