More import removals

Following the series of commits removing deprecated features in
Django 1.7, here are some more unneeded imports removed and other
minor cleanups.

tests/test_utils/tests.py

@@ -1,6 +1,5 @@
 # -*- coding: utf-8 -*-
 from __future__ import absolute_import, unicode_literals
-import warnings
 
 from django.db import connection
 from django.forms import EmailField, IntegerField