BUG: Fix bug with symmetric difference of two equal MultiIndexes GH12490
- [x] closes #13490
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Fixes a bug where the symmetric difference of two equal MultiIndexes would raise a TypeError. MultiIndex used to use the `Index.symmetric_difference`. With this PR it it's own implementation that is more like `MultiIndex.difference`.
