BUG: GH12896 where extra elements are returned in MultiIndex slicing
- [x] closes #12896
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Is this the right way to do this?

cc @MaximilianR 

pls update when you can.
 Will do tonight!
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13117?src=pr) is **84.14%**

> Merging [#13117](https://codecov.io/gh/pydata/pandas/pull/13117?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **+<.01%**

``` diff
@@             master     #13117   diff @@
==========================================
  Files           138        137     -1   
  Lines         50384      50287    -97   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42382      42312    -70   
+ Misses         8002       7975    -27   
  Partials          0          0          
```
1. 2 files (not in diff) in `pandas/io` were modified. [more](https://codecov.io/gh/pydata/pandas/commit/94a6beda94cbb38505f0a892fa58a0e21bd3e660/changes?src=pr#70616E6461732F696F) 
   - Misses `+57` 
   - Hits `+3`
2. 5 files (not in diff) in `pandas/core` were modified. [more](https://codecov.io/gh/pydata/pandas/commit/94a6beda94cbb38505f0a892fa58a0e21bd3e660/changes?src=pr#70616E6461732F636F7265) 
   - Misses `-16` 
   - Hits `-69`
3. 4 files (not in diff) in `pandas` were modified. [more](https://codecov.io/gh/pydata/pandas/commit/94a6beda94cbb38505f0a892fa58a0e21bd3e660/changes?src=pr#70616E646173) 
   - Misses `-2` 
   - Hits `-5`
4. File `...das/indexes/multi.py` was modified. [more](https://codecov.io/gh/pydata/pandas/commit/94a6beda94cbb38505f0a892fa58a0e21bd3e660/changes?src=pr#70616E6461732F696E64657865732F6D756C74692E7079) 
   - Misses `0` 
   - Partials `0` 
   - Hits `+1`
5. File `pandas/io/s3.py` (not in diff) was deleted. [more](https://codecov.io/gh/pydata/pandas/commit/94a6beda94cbb38505f0a892fa58a0e21bd3e660/changes?src=pr#70616E6461732F696F2F73332E7079)

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [01dd111...94a6bed](https://codecov.io/gh/pydata/pandas/compare/01dd11109a0d1def8bc3b03d06c533817cc273f2...94a6beda94cbb38505f0a892fa58a0e21bd3e660)
 ty sir!
