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

diff --git a/tests/test__auth.py b/tests/test__auth.py
index 6347e42..08aa83c 100644
--- a/tests/test__auth.py
+++ b/tests/test__auth.py
@@ -12,13 +12,13 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import unittest
 import mock
 
 import google.auth.credentials
 import google_auth_httplib2
 import httplib2
 import oauth2client.client
-import unittest2 as unittest
 
 from googleapiclient import _auth