Index.difference should allow for None values in the "other" list

[svetlin-mladenov]

Currently the type annotation for Index.difference is

def difference(self, other: Union[List[T1], Index]) -> Index: ...

None values are disallowed for the other argument if it is a list. As a result of that the following code, which runs perfectly fine, will give an error when mypy is used to check it:

from pandas.core.indexes.api import Index

index = Index(["a", "b", "c"])
index.difference(["a", None])

I think the function type should be changed to:

def difference(self, other: Union[List[Optional[T1]], Index]) -> Index: ...

[Dr-Irv]

Thanks @svetlin-mladenov for the report. This is fixed in #18 and now in main