[red-knot] Use try_call_dunder for augmented assignment

[sharkdp]

Summary

Uses the try_call_dunder infrastructure for augmented assignment and fixes the logic to work for types other than Type::Instance(…). This allows us to infer the correct type here:

x = (1, 2)
x += (3, 4)
reveal_type(x)  # revealed: tuple[Literal[1], Literal[2], Literal[3], Literal[4]]

Or in this (extremely weird) scenario:

class Meta(type):
    def __iadd__(cls, other: int) -> str:
        return ""

class C(metaclass=Meta): ...

cls = C
cls += 1

reveal_type(cls)  # revealed: str

Union and intersection handling could also be improved here, but I made no attempt to do so in this PR.

Test Plan

New MD tests

[github-actions]

mypy_primer results

No ecosystem changes detected ✅

[AlexWaygood]

Fantastic!