Add Annotations to GroupBy
Not fully complete but got started on these on a flight today. Should be able to review failures in a few more details.

@gwrome @vaibhavhrt would love any feedback from you
Hello @WillAyd! Thanks for opening this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:







* In the file [`pandas/core/groupby/groupby.py`](https://github.com/pandas-dev/pandas/blob/d89a75d6b70a0c433d9fe3e6573d1975bf2b0271/pandas/core/groupby/groupby.py):

> [Line 1640:45](https://github.com/pandas-dev/pandas/blob/d89a75d6b70a0c433d9fe3e6573d1975bf2b0271/pandas/core/groupby/groupby.py#L1640): [E222](https://duckduckgo.com/?q=pep8%20E222) multiple spaces after operator

 Will come back to this at a later date