Removed unused import.

tests/mail/test_sendtestemail.py

@@ -1,11 +1,9 @@
-# -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
 from django.core import mail
 from django.core.management import call_command
 from django.core.management.base import CommandError
 from django.test import SimpleTestCase
-from django.utils.six import StringIO
 
 
 class SendTestEmailManagementCommand(SimpleTestCase):