gh-112087: Make list.extend(dict) behave atomically

Add a special case for `list.extend(dict)` and `list(dict)` so that
those patterns behave atomically with respect to modifications to the
list or dictionary.

This is required by multiprocessing, which assumes that
`list(_finalizer_registry)` is atomic.

Author: colesbury

Objects/listobject.c

@@ -1374,6 +1374,11 @@ _list_extend(PyListObject *self, PyObject *iterable)
         res = list_extend_set(self, (PySetObject *)iterable);
         Py_END_CRITICAL_SECTION2();
     }
+    else if (PyDict_CheckExact(iterable)) {
+        Py_BEGIN_CRITICAL_SECTION2(self, iterable);
+        res = list_extend_dict(self, (PyDictObject *)iterable, 0 /*keys*/);
+        Py_END_CRITICAL_SECTION2();
+    }
     else if (Py_IS_TYPE(iterable, &PyDictKeys_Type)) {
         PyDictObject *dict = ((_PyDictViewObject *)iterable)->dv_dict;
         Py_BEGIN_CRITICAL_SECTION2(self, dict);