Allow Mypy/Zuban to ignore specific error codes

[davidhalter]

Changes the error code from additional_stuff to assignment (as Mypy uses it).

This was the proposal Rebecca:

For # type: ignore[additional_stuff], I can see the argument for allowing an error about additional_stuff being an invalid code, but it seems tricky to update that line to allow both mypy’s and pyright’s behavior, since they’re so different. (Mypy also reports an assignment error, since that error code isn’t suppressed.) Maybe we could change it to:

z: int = "" # type: ignore[assignment]

so that the expected behavior in all type checkers is that there’s no error?

[JelleZijlstra]

This will need changes in the spec text as well as the conformance suite, and it will need to be approved by the Typing Council. (Personally, I'm supportive of doing something like this, but haven't thought too much about the exact proposal yet.)