os was already imported in the module, no need to re-import

Lib/mimetools.py

@@ -112,7 +112,6 @@ def choose_boundary():
     import random
     if _prefix is None:
         import socket
-        import os
         hostid = socket.gethostbyname(socket.gethostname())
         try:
             uid = `os.getuid()`