This switches us to using pytest for running unit tests, along with a

couple of suitable plugins. pytest is maintained, as a wide ecosystem of

plugins, and offers us options for rethinking our testing infrastructure

down the road.
For the moment, we're still building unit tests around Python's

unittest library, Django's django.test, and Djblets's

djblets.testing. We'll probably continue to do this for the

foreseeable future, since there are benefits to the unittest-based

approach still, and switching everything over to pytest's view of

testing is a big project.
The configuration has a number of warnings that we disable. As we finish

the move to Django 3.2, we'll be able to remove many of these.
One important thing to note is that the ./reviewboard/manage.py test

command will still attempt to use the old test runner. Extensions use

that test runner as well, making that a tricky thing to cleanly switch

out.
So technically, we currently have two test runners we're using, but

pytest will be the official way to run the test suite for Review Board

going forward.

Ran pytest.
Unit tests are currently broken on release-5.0.x, but pytest at least

has the same errors (at least until I had to kill the suite).
This change was originally on release-4.0.x, where it passed the entire

suite.