BUG: Stack/unstack do not return subclassed objects (GH15563)
- [x] closes #15563
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Basically picked from #15655 to support stack/unstack in subclassed DataFrames and Series.
