Add Django 3.1, 3.2, 4.0, 4.1, remove old versions, update deps

[rixx]

Thank you for providing Classy-Django-Forms! I noticed it's a bit out of date, tried to update to include the latest four Django versions, and fixed what I encountered in the process.

Note that I removed Django v1.8 – 1.11 (so all <2.0) in the process, since those have been deprecated for a long time and won't build with modern Python anymore at all.

I can make a PR against the gh-pages branch too, if you'd like.