[mypyc] Fix multiple inheritance with a protocol on Python 3.12 (#15572)

Fix the testProtocol test case in mypyc/test-data/run-classes.test.

There was a comment indicating that the code that causes the test to
fail on 3.12 is required Python versions older than 3.7, so let's remove
it. We only support 3.8 and later these days.

Work on mypyc/mypyc#995.

mypyc/lib-rt/misc_ops.c

@@ -177,42 +177,6 @@ PyObject *CPyType_FromTemplate(PyObject *template,
     if (!name)
         goto error;
 
-    // If there is a metaclass other than type, we would like to call
-    // its __new__ function. Unfortunately there doesn't seem to be a
-    // good way to mix a C extension class and creating it via a
-    // metaclass. We need to do it anyways, though, in order to
-    // support subclassing Generic[T] prior to Python 3.7.
-    //
-    // We solve this with a kind of atrocious hack: create a parallel
-    // class using the metaclass, determine the bases of the real
-    // class by pulling them out of the parallel class, creating the
-    // real class, and then merging its dict back into the original
-    // class. There are lots of cases where this won't really work,
-    // but for the case of GenericMeta setting a bunch of properties
-    // on the class we should be fine.
-    if (metaclass != &PyType_Type) {
-        assert(bases && "non-type metaclasses require non-NULL bases");
-
-        PyObject *ns = PyDict_New();
-        if (!ns)
-            goto error;
-
-        if (bases != orig_bases) {
-            if (PyDict_SetItemString(ns, "__orig_bases__", orig_bases) < 0)
-                goto error;
-        }
-
-        dummy_class = (PyTypeObject *)PyObject_CallFunctionObjArgs(
-            (PyObject *)metaclass, name, bases, ns, NULL);
-        Py_DECREF(ns);
-        if (!dummy_class)
-            goto error;
-
-        Py_DECREF(bases);
-        bases = dummy_class->tp_bases;
-        Py_INCREF(bases);
-    }
-
     // Allocate the type and then copy the main stuff in.
     t = (PyHeapTypeObject*)PyType_GenericAlloc(&PyType_Type, 0);
     if (!t)