BUG: Series.isin fails or categoricals
 - closes #16639 
 - [1 ] tests added / passed
 - ] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff`` (On Windows, ``git diff upstream/master -u -- "*.py" | flake8 --diff`` might work as an alternative.)
 - [check ] whatsnew entry
