asyncio: Replace "unittest.mock" with "mock" in unit tests

Use "from unittest import mock". It should simplify my work to merge new tests
in Trollius, because Trollius uses "mock" backport for Python 2.

Lib/test/test_asyncio/test_queues.py

@@ -1,7 +1,7 @@
 """Tests for queues.py"""
 
 import unittest
-import unittest.mock
+from unittest import mock
 
 import asyncio
 from asyncio import test_utils
@@ -72,7 +72,7 @@ class QueueBasicTests(_QueueTestBase):
         self.assertTrue('_queue=[1]' in fn(q))
 
     def test_ctor_loop(self):
-        loop = unittest.mock.Mock()
+        loop = mock.Mock()
         q = asyncio.Queue(loop=loop)
         self.assertIs(q._loop, loop)