Specify what happens for binary operators (e.g. `__add__`)

There's a subtle dance around __add__, __radd__, and NotImplemented. While most of the rules are in the language spec, I think we haven't specified carefully what the signature should be of e.g. the __add__ method, especially when annotations are inline. (At least, I didn't find any mention of NotImplemented in the spec.)

In typeshed we seem to ignore the "overload" __add__: (Any) -> Literal[NotImplemented] (to be loose with notation), but what do typecheckers do when they see a concrete __add__ or __eq__ implementation that can return NotImplemented?

まず、add、radd、eq、NotImplemented に関する言語仕様のルールを確認し、次に typeshed が現在これらのメソッドをどのように表現しているかを比較します。NotImplemented を返す可能性がある具象実装を、型チェッカーがどのように扱うかを確認します。指定すべきシグネチャと動作について合意に達すれば完了です。