Make the dependency to setuptools explicit for python > 3.12

[Pierre-Sassoulas]

pkg_resources is imported and it's inside setuptools which breaks under python 3.12.

File /venv/lib/python3.12/site-packages/example.py, line 5, in <module>
  import pepper
File /venv/lib/python3.12/site-packages/pepper/__init__.py, line 4, in <module>
  import pkg_resources
ModuleNotFoundError: No module named 'pkg_resources'

There's an easy workaround (installing setuptools ourselves), but it would be nice to release a new version for this to be handled by pip.

[barneysowood]

@Pierre-Sassoulas thanks for this - I'm going to close as I believe this should be resolved by #236. Please let me know if not.