Modernize CI setup, test Django 4.2 and Python 3.11

[bittner]

Makes the tests run against Django 4.2 (dropping 4.0) and Python 3.11, in addition.

Python 3.6 is not available anymore in GitHub's ubuntu-latest VM image, hence we must specify to the older image to be used for running our tests.

PyClean now has the --debris and --erase options that allow us to get all cleanup tasks done from a single tool, finally.

References

• About GitHub-hosted runners (GHA docs)
• Django releases (officially supported versions)
• Supported Python versions: Django 3.2, Django 4.1, Django 4.2 (relevant for the test matrix in tox.ini)