Add GitLab CI and make sure it works with Django 2.2, 3.0, and 3.1

We are relying on this repo for almost all our django projects and the boilerplate but this has no CI or hasn't been updated to work with django > 2.2.

Add CI to check that the library pass tests with current django versions supported (2.2, 3.0, and 3.1) with the last version of Python 3.