Use string.ascii_letters instead of string.letters (SF bug #226706).
diff --git a/Tools/idle/UndoDelegator.py b/Tools/idle/UndoDelegator.py
index ec7af81..3ef14c3 100644
--- a/Tools/idle/UndoDelegator.py
+++ b/Tools/idle/UndoDelegator.py
@@ -251,7 +251,7 @@
         self.chars = self.chars + cmd.chars
         return 1
 
-    alphanumeric = string.letters + string.digits + "_"
+    alphanumeric = string.ascii_letters + string.digits + "_"
 
     def classify(self, c):
         if c in self.alphanumeric: