Drops support for Python 2, Django 1.11, and 2.2, and adds support for Django 4

[renatooliveira]

Description

closes #77

This PR drops support for Python 2, Django 1.11, and 2.2, and adds support for Django 4.

• Tox for some reason wasn't running ./runtests.py, I had to add python instead of running the executable
• DEFAULT_AUTO_FIELD is now good to have in settings

Todos

• [ ] Tests
• [x] Documentation
• [ ] Changelog