bpo-28869: Set up inheritance registry in ABCMeta.__init__. by serhiy-storchaka · Pull Request #16613 · python/cpython

This is strictly speaking not 100% backwards compatible change. Some subclasses of ABCMeta may override __new__ and may depend on the fact that abstract status and/or caches are ready after calling super().__new__(...) (IIRC this was the situation with typing.GenericMeta at some point when it existed).

So I would not backport this to older versions (also maybe this behaviour should be clarified in the docs), otherwise looks good.