DEPR: DataFrame GroupBy indexing with single items DeprecationWarning


- [x] closes #23566
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

theres another FutureWarning you need to catch in test_getitem_numeric_column_names Hello @yehoshuadimarsky! 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 2020-01-02 04:05:51 UTC thanks @yehoshuadimarsky very nice Thanks! My first pandas PR, pretty cool