Django 4 support

[marksweb]

This brings django 4.x support, including testing against django's main branch.

It also moves the github action unittest workflow to run tox so testing can be consistent and hopefully completely reproducible between development and CI. The only downside to this test setup is that the workflows are less verbose. On here we simply see each python version defined in the test workflow, and tox will run everything defined for that python version. It also moves the coverage upload out of the test steps to be an independent section that depends on the tests having passed first.

It drops django 2.2, so we'll need to move to a new major version and leave a support branch for where master is now.

[fsbraun]

Shall we add django CMS 4.0 and 4.1 to the classifiers?

[marksweb]

@fsbraun I will if I can get the checks passing 😬😂