Django compat: 3.1, 3.2, 4.0

[ewjoachim]

This PR:

• Adds Django 3.1, 3.2, 4.0 compatibility
• Adds Python 3.10 and removes 3.6
• Switches from Travis to GitHub Actions
• Adds Black, isort, pytest
• Corrects the doc that was largely false
• Removed the useless makefile
• Fixed the demo which was installed through a fake package with its own setup.py. Now the demo is just a module.
• Adds autorelease on PyPI through GitHub Actions when publishing a GitHub Release (and remove the Changelog)

It's probably more readable if you read it commit per commit, so that you can skip the black & isort commits.

[ewjoachim]

Crazy PR, sorry for putting together a lot of things, but I have limited time, and want to make this a bit more futureproof.