is_subtype vs is_proper_subtype

[ilevkivskyi]

Currently, we have two functions is_subtype and is_proper_subtype. There are two corresponding visitors: SubtypeVisitor and ProperSubtypeVisitor, both of them implement some complex logic.
I think that ideally there should be only one visitor, ProperSubtypeVisitor, and is_subtype should be simply:

def is_subtype(left, right):
    if isinstance(left, AnyType) or isinstance(right, AnyType):
        return True
    return is_proper_subtype(left, right)

Are there any reasons why it is not possible now? Maybe this question already exists in a roadmap mentioned in #3194 (btw @JukkaL @ddfisher is it possible to make it public?)

(I have found this issue #2038, but I am not sure it is up to date.)

[JukkaL]

It's more complex than that, since List[str] is considered a subtype of List[Any], etc. Maybe we could have a flag for SubtypeVisitor that indicates whether we should be taking a proper or normal subtype. I thought about merging the two when writing the proper subtype visitor, but it didn't seem obvious at that time that combining would make the code easier to read or maintain. However, it might well be worth it, since they duplicate a lot of logic.