Needs code coverage calculation

[dancergraham]

relates to #501

It would be great to have a code coverage calculation in the repo. My first thoughts on this:

• add coverage.py as a test dependency?
• configure coverage to check appropriate files e.g.:
  • exclude docs or report them separately?
  • exclude test_app or report it separately?
  • Any other standard config or exclusions for django projects?
• add a coverage section to the tox configuration, probably just using the latest supported versions of python and django?
• ensure this runs properly in ci - add necessary secrets or env vars
• [if-minor]add a badge to the main project readme either linked to the github workflow or linked to codecov
• [if-minor]ensure it runs properly in codecov - needs config like this ? https://github.com/jazzband/django-axes/blob/master/codecov.yml