fixes for Django 4.0, drop Python 2 support, add GitHub testing

[PetrDlouhy]

Fix timedelta field to be working in Django 4.0. Mostly problems with importing six which was dropped from Django.
I don't think we should keep Python 2 support anymore, so I am rewriting this directly into Python 3 implementations.

[PetrDlouhy]

Tests are now passing in all Python/Django versions configured in TOX (Python 3.7-3.10, Djagno 1.8-4.1).

[PetrDlouhy]

I have added GitHub actions test configuration. So this now should run tests when some maintainer will allow it to run (@schinckel @yotamofek ?).

[yotamofek]

@PetrDlouhy hey there,
I'm not the owner of this repo, I used to work for the start up that owned the org this repo is in, but I doubt anyone would still have access to it.
I probably forked this repo from somewhere but it's been way too long ago (8 years!!) for me to remember. I definitely didn't write it myself
I'd recommend you look for the original repo through some of the other commiters' profiles. Good luck!