Support later Django versions
Functionally this MR just bumps the minimum Django version in the requirements.txt
file so that we can make use of it in Django 4 projects.
While updating the .gitlab-ci.yml
file I saw that it had to jump through some horrible hoops to support matrix testing with Django versions and Python versions.
GitLab now supports this feature natively so move over to the common pipeline (docs) which lets us do proper matrix testing with tox.
We now test versions of Django 3.2 through 4.2 with the "modern" Python versions defined by our CI template.
Closes #1 (closed)