Use string.ascii_letters instead of string.letters (SF bug #226706).
diff --git a/Lib/ntpath.py b/Lib/ntpath.py
index 13de59b..d81e8fb 100644
--- a/Lib/ntpath.py
+++ b/Lib/ntpath.py
@@ -334,7 +334,7 @@
     if '$' not in path:
         return path
     import string
-    varchars = string.letters + string.digits + '_-'
+    varchars = string.ascii_letters + string.digits + '_-'
     res = ''
     index = 0
     pathlen = len(path)