tests: drop use of 'unittest2' (#1564)

diff --git a/tests/test_mocks.py b/tests/test_mocks.py
index d0a0ff0..287da95 100644
--- a/tests/test_mocks.py
+++ b/tests/test_mocks.py
@@ -24,7 +24,7 @@
 
 import httplib2
 import os
-import unittest2 as unittest
+import unittest
 
 from googleapiclient.errors import HttpError
 from googleapiclient.errors import UnexpectedBodyError