fix to is_string
- [X] closes #34295
- [x] tests added / passed
- [X] passes `black pandas`
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

Can you add a test? Also I think may have linked to the wrong issue? ahh thank you @WillAyd, transcription error on my part, I've updated the associated issue and added a unit test. @TomAugspurger  Thanks!