DEPR: make Categorical.ravel() return Categorical
Discussed in #27142.

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
conflict and failing

```
pandas/tests/extension/test_categorical.py(100,): error :  Found unwanted pattern:         with pytest.warns(FutureWarning, match="will return a Categorical")
```
otherwise lgtm. I am a little concerned by the change in testing pattern this introduces. Could we remove this for now. testing pattern reverted lgtm. merge on green. thanks @jbrockmendel 