CLEAN: Fix up Groupby classes #149
Description
In pandas 1.1., the classes were _GroupBy and Groupby. Since that time, _Groupby was replaced by BaseGroupBy, and generics were introduced.
The original version of these stubs were generated on top of pandas 1.1.
This should get reconciled at some point so the class names are the same, including the various subclassing that is going on.
In pandas, BaseGroupBy inherits from SelectionMixin[NDFrameT], but if that is used in the stubs, mypy complains because the definitions of aggregate() are different in SelectionMixin and GroupBy
So there is some work to do here.