Use "from . import" for relative imports

Will fail in python3 otherwise.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/folder/IMAP.py

@@ -21,7 +21,7 @@ import binascii
 import re
 import time
 from sys import exc_info
-from Base import BaseFolder
+from .Base import BaseFolder
 from offlineimap import imaputil, imaplibutil, OfflineImapError
 from offlineimap.imaplib2 import MonthNames