Decide on a one true workflow for using polytracker

[surovic]

We should probably still run tests with a couple Python versions though just in case.

Hmm, should we really? We essentially dictate the python version in the docker image build.

My reasoning here is that we have in the past allowed the use of different Python versions with Polytracker. If someone out there is using it with a prior Python version, it would be good to not arbitrarily take that support away from them. However, we only support Polytracker usage in the container, to the best of my knowledge, and as long as we're clear there's one true way to run Polytracker in the README, I don't see any reason to continue running builds or tests under any Pythons other than the Python version we prefer to build and run with.

Originally posted by @kaoudis in #6564 (comment)