Setup.py is deprecated

[tonial]

Following #539 I tried to replace PYTHONWARNINGS=default with PYTHONWARNINGS=error in tox.ini (since it's what I do in my own projet that uses mozilla-django-oidc) and it failed on the use of setuptools :

setuptools.warnings.SetuptoolsDeprecationWarning: setup.py install is deprecated.
      !!
      
              ********************************************************************************
              Please avoid running ``setup.py`` directly.
              Instead, use pypa/build, pypa/installer, pypa/build or
              other standards-based tools.
      
              See https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html for details.
              ********************************************************************************

I don't thing I'll find the time to fix this, but I think it's worth an issue.