BUG: Fix for GH #14848 for groupby().describe() with tuples as the Index
 - [x] closes #14848 
 - [x ] tests added / passed
 - [x ] passes ``git diff upstream/master | flake8 --diff``
 - [x ] whatsnew entry

This isn't the most elegant fix for the bug, but all the tests pass.  There is probably a bigger issue to deal with here, which is to support tuples as an `Index` (as opposed to a `MultiIndex`) throughout, but at least this fix clears the bug.