BUG: pivot_table with margins=True fails for categorical dtype, #10989
This is a fix for the issue reported in #10989. I suspect this is an example of "fixing the symptom" rather than "fixing the problem", but I think it makes clear what the source of the problem is: to compute margins, the pivot table must add a row and/or column to the result. If the index or column is categorical, a new value cannot be added.

Let me know if you think there are better approaches to this.
