F841 is not checked since Train

[Matt Riedemann]

With this change in Train:

https://review.opendev.org/#/c/651553/

We're using pycodestyle rather than pep8 and pycodestyle doesn't check for F841:

https://pycodestyle.readthedocs.io/en/latest/intro.html#error-codes

Which means things like this slip through and then fail on backports to <train:

https://review.opendev.org/#/c/691282/

It would be good to see if we can get F841 checked on master and train again so we don't have that difference with stable branches. I'm not sure how trivial that is though, like if we can separately run flake8 with F841 during our pep8 runs.

This is just to track the issue but I realize it might be closed as Won't Fix.