Bug in Series.groupby would raise ValueError when grouping by PeriodIndex level
- [ ] closes #34010
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I think all comments addressed. will merge tomorrow if no further comments. Opened https://github.com/pandas-dev/pandas/issues/34240 for the followup. LGTM.