implement non-controversial cleanup portions of #18762
#18762 touches a lot of code and is currently in limbo pending discussion of `roll_monthday` and `roll_convention`.  This separates out unrelated cleanup portions of that PR, the idea being that making the diff there simpler will make things easier.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
