>>>I guess a PR to fix the registry output would make sense (first file a bug onbugs.python.org for it).
>>
>>Ok, I will!
>>
>>
>>
>Please don't hurry with this. I am going to rewrite ABCMeta in C soon.
>In fact most of the work is done but I am waiting for implementation of PEP 560 to settle (need few more days for this).
>
>In the C version the caches/registry will be simpler and will not use WeakSet (instead they will be thin C wrappers around normal sets).

Ok, no problem.

Found out myself why the registry's empty:
every iterator passed to Iterator.register() defines __iter__ and __next__, so they satisfy Iterator.__subclasshook__ and are added to the cache beforehand.