Change the list of CI runs.
This commit changes the testing matrix. This is the general philosophy for testing:
- We need one test per Python version we support.
- We need one test per Django versions we support.
- The above two variants don't need to be with each combination, we simply use a test case with one-supported Python and one-supported Django.
- Test with git-head of Django is allowed to fail.
- Test with git head of dependent packages, mailmanclient and django-mailman3.