gh-145685: Stop the world when updating MRO of existing types

We already have a stop-the-world pause elsewhere in this code path
(type_set_bases) and this makes will make it easier to avoid contention
on the TYPE_LOCK when looking up names in the MRO hierarchy.

Also use deferred reference counting for non-immortal MROs.

Author: colesbury

Objects/typeobject.c

@@ -647,7 +647,6 @@ clear_tp_bases(PyTypeObject *self, int final)
 static inline PyObject *
 lookup_tp_mro(PyTypeObject *self)
 {
-    ASSERT_NEW_TYPE_OR_LOCKED(self);
     return self->tp_mro;
 }
 
@@ -664,8 +663,19 @@ set_tp_mro(PyTypeObject *self, PyObject *mro, int initial)
             /* Other checks are done via set_tp_bases. */
             _Py_SetImmortal(mro);
         }
+        else {
+            PyUnstable_Object_EnableDeferredRefcount(mro);
+        }
+    }
+    if (!initial) {
+        type_lock_prevent_release();
+        types_stop_world();
     }
     self->tp_mro = mro;
+    if (!initial) {
+        types_start_world();
+        type_lock_allow_release();
+    }
 }
 
 static inline void
@@ -1728,18 +1738,11 @@ static PyObject *
 type_get_mro(PyObject *tp, void *Py_UNUSED(closure))
 {
     PyTypeObject *type = PyTypeObject_CAST(tp);
-    PyObject *mro;
-
-    BEGIN_TYPE_LOCK();
-    mro = lookup_tp_mro(type);
+    PyObject *mro = lookup_tp_mro(type);
     if (mro == NULL) {
-        mro = Py_None;
-    } else {
-        Py_INCREF(mro);
+        Py_RETURN_NONE;
     }
-
-    END_TYPE_LOCK();
-    return mro;
+    return Py_NewRef(mro);
 }
 
 static PyTypeObject *find_best_base(PyObject *);