BUG: Indexing MultiIndex with Series failed.
Previously, accessing elements of a MultiIndex-indexed DataFrame with a Series
failed. This changes that behavior so that it is possible to use a Series to
access elements from a MultiIndex-indexed DataFrame, just as one would use
a list.

 - [x] closes #14730
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry