Add tests and support for Django 3.2, and tentative tests and support for Django 4.0

[thibaudcolas]

Description

This updates our testing matrix to test in Django 3.2 across all Python versions, and in Django 4.0 across Python versions that are supported.

Django 4.0 removes the long-deprecated django.conf.urls, so I’ve also updated the urlpatterns to use the path and re_path replacements.

I chose not to have the Django 4.0 CI builds as experimental / allowed to fail, as it would require splitting the build into even more separate jobs which I’m not a fan of.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added an appropriate CHANGELOG entry

[bcdickinson]

LGTM, thanks Thibaud