Proposal: use pytest for running tests

[alastair]

I now use pytest in almost all of my projects, including the django ones. This isn't an absolutely necessary change, but there are some things that I like about pytest:

• The syntax for running just one file or one test is a bit easier to remember
• It reports failures a bit more clearly
• you can use basic assert x == y instead of needing to use self.assertEqual(x, y) (nice to have but not a strong requirement)
• you don't need to write tests as a class, you can use any function (nice to have but not a strong requirement)
• Fixtures are a nice way of having reusable functions
• Markers are pretty cool

Because the configuration for the settings file is now in pyproject.toml, to run tests you just need to

pytest

Anyway, this is a proposal to see if we think that it's useful. If so, we can also update the github actions to use it and change the documentation.