FIX: Allow aggregate to return dictionaries again #16741
Fixes a regression from 0.19 to 0.20. The function passed to `aggregate` is allowed again to return dictionaries.
 
- [X] closes #16741
- [X] tests added / passed, added `test_cython_agg_return_dict`
- [X] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
- [X] whatsnew entry
