Switch pydiffx's test runner to pytest.
Review Request #12609 — Created Sept. 19, 2022 and submitted
This switches us entirely over to pytest, replacing the old nose test
runner. We've been using pytest for development, but haven't made it
officially a part of the codebase.
tests/runtests.pyremains, but now wraps pytest, warning of
Unit tests pass on all supported versions of Python.