Besides, defining an ABC for a single, module-specific method sounds rather overkill. This reminds of me of projects plagued by an overuse of interfaces for every possible concept.
Thats what Iterator did in ABC though.
Well, it could be argued that testing for an Iterator is useful for a significant variety of code, while testing for a JSONSerialiazable doesn't have an use case outside of the json module itself.
Besides, it remains to be seen if anyone will use the Iterator ABC instead of directly looking up the __next__ method. I'm not convinced that all of the ABCs bundled with the stdlib are really useful, apart from showcasing the potentialities of ABCs.