explicitly define symbols to import instead of 'import *'

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

offlineimap/imapserver.py

@@ -18,7 +18,7 @@
 
 from offlineimap import imaplibutil, imaputil, threadutil, OfflineImapError
 from offlineimap.ui import getglobalui
-from threading import *
+from threading import Lock, BoundedSemaphore
 import thread
 import time
 import hmac