Currently doctest (pywikibot-core-tox-doctest-docker) runs with Python 3.7 but it would be useful to run doctest with other Python versions too e.g. Python 3.9 like we have it with flake8 and hacking tests (pywikibot-core-tox-docker). Because the current doctest needs 40 seconds only it would be applicable to to run it with several Python versions sequentially.