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?

首先查看语言规范中关于 add、radd、eq 和 NotImplemented 的规则，然后比较 typeshed 当前如何表示这些方法。检查类型检查器如何处理可能返回 NotImplemented 的具体实现。就应指定的签名和行为达成一致即表示完成。