BUG: Use self._constructor_sliced in df._reduce to respect subclassed…
… types. (GH25596)

- [x] closes #25596
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

> Thanks @EmilianoJordan for the PR. will need whatsnew once #30907 is merged.

Subscribed to #30907 to know when I can do this. Also, updated the initial pull request comment to reflect that this has not been completed. thanks @EmilianoJordan 