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

diff --git a/tests/test_json_model.py b/tests/test_json_model.py
index 416b7be..1ddc1c7 100644
--- a/tests/test_json_model.py
+++ b/tests/test_json_model.py
@@ -26,7 +26,7 @@
 import json
 import pkg_resources
 import platform
-import unittest2 as unittest
+import unittest
 import urllib
 
 import googleapiclient.model