pypackage: cache the .tox directory on GHA

[seanh]

The pypackage cookiecutter should cache the .tox directory on GitHub Actions in the same way as the pyramid-app one does. This might speed up CI and tox-recreate should ensure that the venvs get recreated if setup.cfg changes (and tox itself will recreate the venvs if tox.ini changes).

For packages, which use unpinned dependencies, we actually want the venvs on CI to be re-created over time even if tox.ini and setup.cfg don't change so that we test against new versions of our dependencies as they get released. Caching the .tox dir would defeat that. I wonder if we could disable caching just for the scheduled nightly run of ci.yml?

[seanh]

I think we might need to add the Python version to the cache key, otherwise tests running in different versions of Python will fight over the cache.