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

diff --git a/tests/test_errors.py b/tests/test_errors.py
index 955526f..7a2cfde 100644
--- a/tests/test_errors.py
+++ b/tests/test_errors.py
@@ -21,7 +21,7 @@
 __author__ = "afshar@google.com (Ali Afshar)"
 
 
-import unittest2 as unittest
+import unittest
 import httplib2