TST: Remove compat.PY2 and compat.PY3 flags from tests
- [X] xref #25725
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

These flags become trivial after dropping Python 2.  There are still some Python 2 specific things that can be removed from the tests but this should eliminate a lot of the low hanging fruit.

Can split this up into separate PRs if need be; there are lots of changes but they're of low complexity and disjoint from each other, so shouldn't be too bad to review.  Can wait on the other PRs mentioned in the master tracker issue if we want those to go in first.