Use lower name for email module

issue3211

trytond/__init__.py

@@ -2,7 +2,7 @@
 # this repository contains the full copyright notices and license terms.
 import os
 import time
-from email import Charset
+from email import charset
 
 from . import server
 
@@ -13,4 +13,4 @@ if hasattr(time, 'tzset'):
     time.tzset()
 
 # set email encoding for utf-8 to 'quoted-printable'
-Charset.add_charset('utf-8', Charset.QP, Charset.QP)
+charset.add_charset('utf-8', charset.QP, charset.QP)