Don't overwrite empty tp_new when type is inheriting from object

The capi uses this to deny instancing a type directly.

src/capi/typeobject.cpp

@@ -3427,6 +3427,9 @@ extern "C" int PyType_Ready(PyTypeObject* cls) noexcept {
     // RELEASE_ASSERT(cls->tp_traverse == NULL, "");
     // RELEASE_ASSERT(cls->tp_clear == NULL, "");
 
+    // set this flag early because some function check if it is set pretty early
+    cls->is_user_defined = true;
+
     assert(cls->attrs.hcls == NULL);
     new (&cls->attrs) HCAttrs(HiddenClass::makeSingleton());
 #define INITIALIZE(a) new (&(a)) decltype(a)
@@ -3510,7 +3513,6 @@ extern "C" int PyType_Ready(PyTypeObject* cls) noexcept {
         cls->gc_visit = &conservativeAndBasesGCHandler;
     else
         cls->gc_visit = &conservativeGCHandler;
-    cls->is_user_defined = true;
 
 
     if (!cls->instancesHaveHCAttrs() && cls->tp_base) {

test/tests/capi_slots.py

@@ -44,6 +44,14 @@ print slots_test.SlotsTesterNum(0) == slots_test.SlotsTesterNum(1)
 for i in slots_test.SlotsTesterSeq(6):
     print i
 
+try:
+    # seqiter.tp_new is NULL so we should not be allowed to create an instance
+    slot_tester_seqiter = type(iter(slots_test.SlotsTesterSeq(6)))
+    print slot_tester_seqiter
+    slot_tester_seqiter()
+except Exception as e:
+    print e
+
 su = slots_test.SlotsTesterSub(5)
 print su