CLN: Add/refine type hints to some functions in core.dtypes.cast

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

Some cleanup resulting from PR #32894. In order to get SeriesGroupBy._transform_fast to realize self.obj is a Series, it seemed I needed to add a generic type to GroupBy and _GroupBy. I don't know if there is a better way to do this. Here, one cannot use FrameOrSeries as a Series may return a DataFrame and vice-versa, so I added _GroupByT.
> In order to get SeriesGroupBy._transform_fast to realize self.obj is a Series, it seemed I needed to add a generic type to GroupBy and _GroupBy. I don't know if there is a better way to do this. Here, one cannot use FrameOrSeries as a Series may return a DataFrame and vice-versa, so I added _GroupByT.

see https://github.com/pandas-dev/pandas/pull/32302#issuecomment-594541073 thanks @rhshadrach very nice!