Use string.ascii_letters instead of string.letters (SF bug #226706).
diff --git a/Lib/test/test_mimetools.py b/Lib/test/test_mimetools.py
index 4ee11a1..0300714 100644
--- a/Lib/test/test_mimetools.py
+++ b/Lib/test/test_mimetools.py
@@ -2,7 +2,7 @@
 import mimetools
 
 import string,StringIO
-start = string.letters + "=" + string.digits + "\n"
+start = string.ascii_letters + "=" + string.digits + "\n"
 for enc in ['7bit','8bit','base64','quoted-printable']:
     print enc,
     i = StringIO.StringIO(start)