In putting together a patch for this, I noticed that the __subclasshook__ classmethods don't compose well.   I'm unclear about the semantics about what these hooks are supposed to mean for subclasses and whether there is a bug in the existing code for Set, Sequence, and Mapping.  Each of those inherits from Sized, Iterable, and Container, but only the Sized.__subclasshook__ is visible to the subclass.  Also, the code for Sized.__subclasshook__ has an identity check for Sized which doesn't seem correct from the point-of-view of the subclasses.