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

Ideally I'd want to keep the method and change the behavior to (an improved variant of) _internal_get_values, but so it goes.