isinstance(x, mock.Mock) is broken since Mock is aliased to Any

[JukkaL]

#3182 changed mock.Mock to be an alias to Any, which resulted in mypy complaining about isinstance(x, mock.Mock), which I believe should be valid.

We can fix this by reverting #3182 or potentially by making Mock a class with an Any base class. Mock is a class at runtime, so making it an alias to Any (which is not a class) seems wrong to me.

[srittau]

Subclassing sounds to be the best solution to me, if it works. That also means we could add methods to it, which exist on the implementation. Cc @JelleZijlstra

[srittau]

Also MagicMock and AsyncMock (when my Python 3.8 patch gets merged).

[hauntsaninja]

I believe this is fixed by #3846 and others