add an [examples] variant to PyPartMC requirements in setup.py to avoid hacky logic in CI workflow and enable users to pip-install these dependencies

[slayoo]

currently we have this in the workflow file:

          ex -sc 'g/^PyPartMC/d' -cx .binder/requirements.txt
          pip install -r .binder/requirements.txt

and no way for a user to pip-install these dependencies

[github-actions]

Stale issue message