REF: Categorical.is_dtype_equal -> categories_match_up_to_permutation
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

There are too many notions of CategoricalDtype equality.  This gives a clearer name to one of them.