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

diff --git a/tests/test_schema.py b/tests/test_schema.py
index f5fd518..b288908 100644
--- a/tests/test_schema.py
+++ b/tests/test_schema.py
@@ -19,7 +19,7 @@
 
 import json
 import os
-import unittest2 as unittest
+import unittest
 
 from googleapiclient.schema import Schemas