Yeah, I find the need for using the string "A" here a wart. Rather than change the entire semantics of annotations, it feels like a placeholder for this meaning would be better.  E.g.:

class A:

    def __add__(self, other: CLS) -> CLS: ...

A static checker could do the magic of recognizing that special name easily enough (no harder than recognizing the quoted string).  At runtime 'CLS' could either just be a singleton with no other behavior... or perhaps it could be some sort of magic introspection object.  It's more verbose, but you can also spell it now as:

class A:

    def __add__(self, other: type(self)) -> type(self): ...

That's a little ugly, but it expresses the semantics we want.