Add tox as test runner

Tox is useful when it is necessary to test against various versions of the Python interpreter and libraries, etc and have an isolated test environment.
It is also good for not being locked to a specific CI system, only simple install tox and run tox

https://tox.readthedocs.io/en/latest/

A complex tox configuration sample:
https://github.com/lumocodes/django-versatileimagefield/blob/django3.2_tox_github_actions/tox.ini
https://github.com/lumocodes/django-celery-beat/blob/master/tox.ini

If the tests need external applications to run, there is the tox-docker plugin that is capable of running docker containers and stopping them during the testing process. In other words: it is an easy way to have postgresql running
A sample usage of tox-docker
https://github.com/lumocodes/django-timezone-field/blob/master/tox.ini#L46-L67

Start by reviewing the repository's existing test and CI setup, then compare the linked tox.ini examples and the tox documentation. Determine the project's supported Python and library versions and whether external applications such as PostgreSQL are required. Done means the test suite runs in an isolated environment with the tox command and remains usable independently of a specific CI system.