Update Django versions to test

[stianjensen]

This removes EOL python 2.7, and all non-LTS django versions that are out of supported.
Also, Django 4.0 which is now released is added to testing.

[stianjensen]

Will probably need #127 to make tests pass on Django 4.0

[stianjensen]

Coming back to review my own code with a big facepalm:

I've excluded all tests from running 🤦 Must have not seen the exclude keyword when removing the 2.7 entries...

[erdem]

@stianjensen don't worry about your code, this project was integrated into travis-ci.org at the time that I have developed it. I already got migrating issues with travis-ci.com. I thought your changes can trigger a new build on new travis but no luck. I have reverted .travis.yml file and added django.4.0 env to it in the main branch. I will work on this more later.

[stianjensen]

@stianjensen don't worry about your code, this project was integrated into travis-ci.org at the time that I have developed it. I already got migrating issues with travis-ci.com. I thought your changes can trigger a new build on new travis but no luck. I have reverted .travis.yml file and added django.4.0 env to it in the main branch. I will work on this more later.

Okay, I see!

I created a new PR to try and cleanup, but feel free to close that again, then 👍 At this point it's probably better to just migrate to Github Actions instead, anyway.