As with every type, that has been created and initialized, HeapTypes created form PyType_FromSpecWithBases() have to pass through PyType_Ready(). Here the function inherit_special might be called, which, among other things, does the following:
....
3892         if (base != &PyBaseObject_Type ||                                                                                                                
3893             (type->tp_flags & Py_TPFLAGS_HEAPTYPE)) {
3894             if (type->tp_new == NULL)
3895                 type->tp_new = base->tp_new;
3896         }
....
The code does not know of Heaptypes that have been created from extension-types by the PEP 384 refactorings. This includes extension-types that do not specify a tp_new method but instead have seperate factory methods, that are only used within the extension module. inherit_special() might consequently assign inappropriate new-methods to these type objects.

To circumvent this issue, I propose to enhance PyType_FromSpecWithBases in the following way: If no tp_new has been specified, we assign the newly defined PySpec_New() method to tp_new which simply denies the user to create an instance of this type. This also prohibits inherit_special to falsely inherit inappropriate new-methods.