> IMO, we can drop most (all?) of the huge comment before the check in object_set_class().

I don't think so. I don't understand why it's allowed to override the __class__ attribute of ModuleType instances whereas it's a static type. IMO the long comment remains useful and something should be done later to make the condition more generic, rather than checking PyType_IsSubtype(newto, &PyModule_Type) specifically.