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

diff --git a/tests/test_model.py b/tests/test_model.py
index c3c936c..07d2261 100644
--- a/tests/test_model.py
+++ b/tests/test_model.py
@@ -23,7 +23,7 @@
 
 __author__ = "jcgregorio@google.com (Joe Gregorio)"
 
-import unittest2 as unittest
+import unittest
 
 from googleapiclient.model import BaseModel
 from googleapiclient.model import makepatch