BUG: CategoricalIndex.equals casting non-categories to np.nan
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

im hopeful we'll be able to just get rid of _is_dtype_compat entirely, but taking it one step at a time imagine this is the root cause of at least a bug or 2 on our tracker