> CPython only makes these methods class method when a class created.
> If you set __class_getitem__ method after the creation it 
> doesn't work unless you use classmethod decorator manually.

Give the intended use case for __class_getitem__ and that it was replacement for metaclass code with the same effect, the current behavior doesn't seem unreasonable.  That said, I don't see any downside to allowing the method to attach after class creation.

Assigning to Ivan to make the call on this one.