Fix Up Typing in GroupBy
More typing cleanups

Original failures:

```sh
pandas/core/groupby/ops.py:13: error: Module 'pandas._libs' has no attribute 'groupby'
pandas/core/groupby/ops.py:13: error: Module 'pandas._libs' has no attribute 'reduction'
pandas/core/groupby/groupby.py:22: error: Module 'pandas._libs' has no attribute 'groupby'
pandas/core/groupby/groupby.py:329: error: Need type annotation for '_apply_whitelist'
pandas/core/groupby/generic.py:220: error: Incompatible types in assignment (expression has type "Callable[[Arg(Any, 'arg'), VarArg(Any), KwArg(Any)], Any]", base class "SelectionMixin" defined the type as "Callable[[Arg(Any, 'func'), VarArg(Any), KwArg(Any)], Any]")
```