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 I noticed it's a bit out of date, tried to update to include the latest four Django versions, and fixed what I encountered in the process.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.

[x-N0]

Damn, maybe will be a good idea to save the current version in another repo for those working on legacy Django projects.

Keep me posted, tho! Just came here looking for Django 3 version of the classy forms, thanks!

[rixx]

I'm not sure there is too much value in preserving this page in a pre-Python 3 version (but all up to @ana-balica of course). People can still use the archive.org versions, or build this repo themselves (or, if they are really stuck with Python 2 today, most likely will just use the Django docs for those versions).

For reference, Django 2.0, the oldest still included version, was released in 2018 and stopped being supported in 2019. Django 1.11 (LTS) stopped being supported April 1, 2020, more than 2.5 years ago.

[rixx]

deferring to #6, I guess.