BUG: .loc with MultiIndex with tuple in level GH#27591
- [x] closes #27591
- [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

This PR does better than existing but its still ambiguous for pathological cases - hence why I'm floating the idea of deprecating tuples as sequence identifiers for multiple keys and relying only on lists (which can't be used as index keys).

**edit**: actually not even doing that would solve the cases below - because nowhere is a list needed, the below depends on how the `slice(None)` might be automatically added or not based on implied levels. 

### Effect of PR

| Before | After |
| --- | --- |
| ![Screen Shot 2021-07-01 at 15 26 34](https://user-images.githubusercontent.com/24256554/124132141-ca526a80-da80-11eb-8359-c09a6d1eea95.png) | ![Screen Shot 2021-07-01 at 15 26 11](https://user-images.githubusercontent.com/24256554/124132158-d0484b80-da80-11eb-853a-8be2dce90f93.png) | 
 > hence why I'm floating the idea of deprecating tuples as sequence identifiers for multiple keys and relying only on lists (which can't be used as index keys).

I think we should do this too.  This PR is about fixing the bug in the interim. thanks @jbrockmendel, looking forwards to cleanups on this!