Get Tox Working

[rachelhsmith]

This PR addresses issue #37

The aim is to use tox to run tests across all variations of Django versions 2.2 - 4.0 and Python versions 3.8 - 3.10.

This PR will enable:

• tox to run locally by running 'tox' or 'poetry run tox'
• pytest to run locally on it's own by running 'pytest' or 'poetry run pytest'
• tox to run on pull requests
• Pytest to be used as the runner for tox