support narrowing on `a is b` where `b` is a generic

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Description

Code sample in basedpyright playground

def foo[T](a: object, b: T) -> T:
    assert a is b
    return a # error: Type "object" is not assignable to type "T@foo"

doesn't look like this is in the list of supported type guards but i think it should be supported. i also couldn't find an upstream issue but i'm certain this has been raised and rejected already

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related: #391