TYPING: change to FrameOrSeries Alias in pandas._typing
@WillAyd 

I think we should discuss reverting #28173 and only use `TypeVar("FrameOrSeries", bound="NDFrame")` in `core.generic`. perhaps call it `_NDFrameT` to avoid confusion.

This PR is to show the changes required to keep mypy green if we wanted to revert.

It also reverts the change in #29458, see https://github.com/pandas-dev/pandas/pull/29458#discussion_r343733263

`_GroupBy` is defined as a generic class, but not sure about `Grouping`

I've created it as a draft PR, since I think we should wait for more issues to surface going forward to help decide. The number of changes needed at this stage is not significant enough to warrant a hasty decision.