CLN: fix mypy error pandas/tests/plotting/test_backend.py
GH28926, mypy didn't like setting non-existant attribute on module.
Dummy function, circumvented with `__setattr__`

- [x] addresses part of #28926
- [x] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@TomAugspurger mind taking a look at this one? Thanks!