Thank you for implementing it!
My biggest worry/confusion is the same as in the revolution issue: how does this work with subclassing & inheritance, and how that would interact with third-patry extensions.

Great questions, I haven't gone through enough real examples of that to be certain, but my gut feel would be:

• when subclassing a native type with a pure-Python type, you can't override thetp_getinterface slot anyway
• when subclassing with another native type, you have to know what your base class is, and so can look up itstp_getinterface and delegate explicitly
• as with all other native slot implementations, they will only support subclassing if they are written to do so, and can reject subclassing by testing their own type
• also, native type inheritance is a nightmare and you'll sleep better if you just don't do it :)

I'm not sure whether we should be adding guidance to recommend actively rejecting subclasses intp_getinterface implementations, though it probably makes more sense to do that than to try and make every interface implementation go through slow paths in order to support it. Let fast be fast, and let the fallbacks handle subclasses.