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

diff --git a/tests/test_protobuf_model.py b/tests/test_protobuf_model.py
index 4479b13..68e3963 100644
--- a/tests/test_protobuf_model.py
+++ b/tests/test_protobuf_model.py
@@ -22,7 +22,7 @@
 
 __author__ = "mmcdonald@google.com (Matt McDonald)"
 
-import unittest2 as unittest
+import unittest
 import httplib2
 
 from googleapiclient.model import ProtocolBufferModel