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902d86e7 - [red-knot] Do not emit `invalid-return-type` for abstract functions (#16900)

[red-knot] Do not emit `invalid-return-type` for abstract functions (#16900) ## Summary Resolves #16895. `abstractmethod` is now a `KnownFunction`. When a function is decorated by `abstractmethod` or when the parent class inherits directly from `Protocol`, `invalid-return-type` won't be emitted for that function. ## Test Plan Markdown tests. --------- Co-authored-by: Carl Meyer <carl@oddbird.net>