ENH: Add indexing syntax to GroupBy.nth()
- [x] closes #44589
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

Add index notation to nth for lists of integers and slices
Hello @johnzangwill! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:







There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-12-04 16:01:30 UTC thanks @johnzangwill very nice Hi. Is this already implemented into pandas? I have a use case with groupby slice which is similar to this. Thank you!
 > Hi. Is this already implemented into pandas? I have a use case with groupby slice which is similar to this. Thank you!

@emilmirzayev This has been merged to master for release in 1.4, which was scheduled for December 31 2021.