Mark both relations as read-only after a rename

Only one relation was marked as read-only after a rename, which means
that changes to the original relation would affect the other one
as well.

relational/relation.py

@@ -197,6 +197,7 @@ class relation (object):
 
         newt.content = self.content
         newt._readonly = True
+        self._readonly = True
         return newt
 
     def intersection(self, other):