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

diff --git a/tests/test_discovery_cache.py b/tests/test_discovery_cache.py
index 2c3efd8..cd53318 100644
--- a/tests/test_discovery_cache.py
+++ b/tests/test_discovery_cache.py
@@ -18,7 +18,7 @@
 """Discovery document cache tests."""
 
 import datetime
-import unittest2 as unittest
+import unittest
 
 import mock