DataFrameGroupby.boxplot fails when subplots=False
- [x] closes #16748
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

![Screen Shot 2020-10-13 at 8 06 48 PM](https://user-images.githubusercontent.com/9269816/95898874-037e6980-0d90-11eb-85b3-e00fe36611b9.png)
