Update _Py_dict_lookup() assertion

Author: vstinner

Objects/dictobject.c

@@ -1280,7 +1280,10 @@ _Py_dict_lookup(PyDictObject *mp, PyObject *key, Py_hash_t hash, PyObject **valu
     DictKeysKind kind;
     Py_ssize_t ix;
 
-    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(mp);
+    if (!PyFrozenDict_CheckExact(mp)) {
+        _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(mp);
+    }
+
 start:
     dk = mp->ma_keys;
     kind = dk->dk_kind;