Incorrect type for `Series.unstack`

[stephenlrandall]

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.

[Dr-Irv]

Yes, but we need an example that illustrates the issue with unstack(). Can you provide that?

[stephenlrandall]

@Dr-Irv Sure thing --

import pandas as pd

df = pd.DataFrame([[1, 3, 5], [2, 4, 6]])

vector = df.transpose().stack([*range(df.index.nlevels)])

display(vector)

matrix = vector.unstack([*range(vector.index.nlevels // 2)])

display(matrix)

[Dr-Irv]

Thanks for the report. PR welcome. It's an easy fix.