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

diff --git a/tests/test_discovery.py b/tests/test_discovery.py
index 2f3f028..9559bf6 100644
--- a/tests/test_discovery.py
+++ b/tests/test_discovery.py
@@ -35,7 +35,7 @@
 import pickle
 import re
 import sys
-import unittest2 as unittest
+import unittest
 import urllib
 
 from parameterized import parameterized