Cache test requirements in GitHub actions

Currently we install them with tox, which is not cached, and appears to take >1 min of the test time. Should we see if caching could reduce it?

One option is to cache .tox -- e.g. https://github.com/lsst-sqre/safir/issues/2

Another is probably to move off tox. (Per About caching workflow dependencies, GitHub workflows would cache dependencies with minimal configuration effort.)

If we called pip directly rather than isolating in tox, something like this could work:

- uses: actions/setup-python@v4
  with:
    ...
    cache: 'pip'
    cache-dependency-path: |
      mypy-requirements.txt
      build-requirements.txt
      test-requirements*.txt

Note that I'm not actively pitching to reduce reliance on tox, merely bringing it up as a possibility.

Start by reviewing the GitHub Actions workflows and the tox setup, along with mypy-requirements.txt, build-requirements.txt, and test-requirements*.txt. Compare caching .tox with setup-python's pip cache option. Done means the chosen approach reduces repeated test dependency installation while the test workflow remains passing.