improve error message when ssl_version must be set due to the tls_level

Improve the ssl documentation.

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/imaplibutil.py

@@ -22,10 +22,11 @@ import threading
 import socket
 import errno
 import zlib
-import six
 from sys import exc_info
 from hashlib import sha1
 
+import six
+
 from offlineimap import OfflineImapError
 from offlineimap.ui import getglobalui
 from offlineimap.virtual_imaplib2 import IMAP4, IMAP4_SSL, InternalDate, Mon2num