FIX: pivot_table dropna=False drops index level names
closes #12133 
level names are lost on `MultiIndex.from_arrays(cartesian_product(table.index.levels))`

pls add a whatsnew entry (0.18.0 bug fixes)
you also have linting errors
 pls rebase/update
 closed in favor of #12650 
 I'll do my best to learn how to squash commits ASAP.)
