API: CategoricalDtype.__eq__ with categories=None stricter
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

xref #37929

This will allow us to remove special-casing for Categorical in Block._astype, Index.astype, and Categorical.astype.