Describe the bug

As noted in #703, providing a list of ints or strings to unstack leads to a type error:

I've identified a few places (see attachment). However, I am not sure the patch is correct/complete. It is interesting to see that stack is annotated with level: Level | list[Level] = ... while unstack has level: Level = ... while IMHO both should be IndexLabel.

It seems as though the fix in #837 only touched groupby.