Use string.ascii_letters instead of string.letters (SF bug #226706).
diff --git a/Lib/dospath.py b/Lib/dospath.py
index ed35d59..958f9f6 100644
--- a/Lib/dospath.py
+++ b/Lib/dospath.py
@@ -241,7 +241,7 @@
     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)