BUG: fixed issue with mixed type groupby aggregate
Fixes issue #16916, where using aggregate on a mixed type grouping
vector fails. Added test in test_aggregate.py to ensure that the bug
is fixed.

cc @ElDeveloper 

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