CLN remove unnecessary trailing commas in groupby tests
xref #35925

Hello @xcz011! 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 2020-09-02 12:13:40 UTC Thanks @xcz011 