Groupby crash on a single level
- [/] closes #30208
- [/] tests added / passed
- [/] passes `black pandas`
- [/] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @nlepleux! 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 2019-12-13 15:23:08 UTC It is possible to restart the CI process? The git checkout failed on windows all green Great thanks @nlepleux 