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.
Hello @suoniq! 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 2021-09-05 02:10:58 UTC Please note that I'll be on vacation until mid-August and plan to not come close to any computers. So I won't respond Do any necessary changes to the MR if you don't want to wait. See you then. cc @rhshadrach if you can look @suoniq - friendly ping. It'd be great to get this in for 1.3.2 and I think it's almost there. If you don't have time to finish it up, would you mind if I pushed it across the finish line? @simonjayhawkins - I've tagged this as 1.3.2 but certainly don't hold up the release if it doesn't get in by tomorrow.

@suoniq - Thanks for the PR and apologies for the ping above, I missed your note about being out.

cc @jreback @jbrockmendel moving to 1.3.3 Hi, back from vacation.  
@rhshadrach Of course I am fine with you taking over. Still consider the original test as good b/c it pin-pointed exactly the problem: time series behave differently than series indexed differently. Was red before the fix as well. `(-;`  
Anyway, please let me know, if there is anything left I can do. @suoniq I didn't think the intention was to test on empty objects, but let me know if this is incorrect. I think we're in good shape here, just waiting on review. More than happy if you want to take the reins back again! @jreback and @jbrockmendel for review Thanks @jreback - comment addressed and green. thanks @suoniq and @rhshadrach  @meeseeksdev backport 1.3.x Something went wrong ... Please have  a look at my logs.