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: ...

No implementation files, tests, or entry points are identified. Start by reviewing the sentinel-value proposal and its referenced typeshed discussion, then determine which syntax and semantics should be accepted. Done requires an agreed design and corresponding implementation and tests, but the issue does not define those details.