BUG: Series.groupby fails with InvalidIndexError on time series with …
…a tuple-named grouper. (#42731)

- [x] closes #42731
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Skipped any "What's New" for now, since it's not decided which version will have it. Will update as soon as this is decided.