Decide and implement a modern way to run Python tests
setup.py test is deprecated. We should decide and implement a new way to run tests.
Probably the most straightforward migration would be to set up tox. The simplest commands could install our test dependencies and run
python -m unittest.
We may get benefits like nicer test reporting if we switch to a front-end like
pytest. At least some of them natively support unittest-style tests.