TYP: selection and groups type-hinting in groupby
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

can you rebase and ping on green. /azp run <samp>
Azure Pipelines successfully started running 1 pipeline(s).<br>

</samp> @jreback green