Add typing.Reader and typing.Writer

I recently stumbled at a TODO in typeshed that proposes to split IO[AnyStr] into smaller pieces. I think it is a good idea and propose to add two protocols:

class Reader(Protocol[AnyStr]):
    def read(self, n: int = ...) -> AnyStr: ...
    def readlines(self) -> List[AnyStr]: ...
    def close(self) -> None: ...

class Writer(Protocol[AnyStr]):
    def write(self, s: AnyStr) -> int: ...
    def writelines(self, lines: List[AnyStr]) -> None: ...
    def close(self) -> None: ...

Then IO[AnyStr] can subclass both, plus add some less often used methods (also IO should stay nominal). What do you think?

从 typeshed 中引用的 TODO 开始，并审查 typing.IO 的现有定义。将提议的 Reader 和 Writer 协议与 IO 中较少使用的方法，以及 IO 必须保持 nominal 的要求进行比较；完成的标准是就协议设计达成一致，并记录或实现已接受的更改。