I like this proposal; given Python's flat nominal type hierarchy, it will be useful to have a parallel subtyping mechanism to give things finer granularity without having to resort to ABCs.

Are the return types of methods invariant or variant under this proposal?

I.e. if I have

  class A(Protocol):

    def f() -> int: ...

does

  class B:

    def f() -> bool:

      return True

implicitly implement the protocol A?

Also, marking Protocols using subclassing seems confusing and error-prone.

In your examples above, one would think that you could define a new protocol using

class SizedAndClosable(Sized):

    pass

instead of

class SizedAndClosable(Sized, Protocol):

    pass

because Sized is already a protocol.

Maybe the below would be a more intuitive syntax:

  @protocol

  class SizedAndClosable(Sized):

      pass

Furthermore, I strongly agree with #7. Typed, but optional, attributes are a bad idea.