Literal for sentinel values

This came up python/typeshed#3521: Currently I can't think of a way to type sentinel values that are often constructed by allowing a certain instance of object as the argument. For the example above it would be useful to be able to do something like this:

class PSS(AsymmetricPadding):
    MAX_LENGTH: ClassVar[object]
    def __init__(self, mgf: MGF1, salt_length: Union[int, Literal[MAX_LENGTH]]) -> None: ...

Alternatively we could add a type like Singleton type to typing:

class PSS(AsymmetricPadding):
    MAX_LENGTH: Singleton
    def __init__(self, mgf: MGF1, salt_length: Union[int, MAX_LENGTH]) -> None: ...

尚未确定任何实现文件、测试或入口点。首先审阅 sentinel 值提案及其中引用的 typeshed 讨论，然后确定应接受哪些语法和语义。完成的要求是达成一致的设计，并提供相应的实现和测试，但该 issue 未定义这些细节。