Test: use tox4 to run the tests

[humitos]

Define https://tox.wiki/en/4.16.0/config.html#skip_missing_interpreters so when running on CircleCI it only runs the environments for the Python versions we have defined.

Closes #243

[humitos]

This approach could work. However, CircleCI images contain more than 1 Python version. This job https://app.circleci.com/pipelines/github/readthedocs/sphinx-notfound-page/317/workflows/74d3a857-361d-4e05-880c-62325eac77ac/jobs/1685 executes Python 3.9 and 3.10 😞

  py39-sphinx5: OK (16.12=setup[10.67]+cmd[5.45] seconds)
  py39-sphinx6: OK (15.69=setup[9.87]+cmd[5.83] seconds)
  py39-sphinx7: OK (15.78=setup[10.27]+cmd[5.50] seconds)
  py310-sphinx5: OK (14.36=setup[9.11]+cmd[5.25] seconds)
  py310-sphinx6: OK (14.14=setup[8.88]+cmd[5.26] seconds)
  py310-sphinx7: OK (15.18=setup[9.84]+cmd[5.35] seconds)
  py310-sphinx8: OK (15.09=setup[9.13]+cmd[5.96] seconds)
  py310-sphinxlatest: OK (15.81=setup[10.42]+cmd[5.39] seconds)
  py310-sphinxdev: OK (17.88=setup[11.55]+cmd[6.33] seconds)