Update tests to use unittest2.
diff --git a/tests/test_bigfile.py b/tests/test_bigfile.py
index 39bd095..02e052e 100644
--- a/tests/test_bigfile.py
+++ b/tests/test_bigfile.py
@@ -4,12 +4,12 @@
     from StringIO import StringIO
 except ImportError:
     from io import StringIO
-import unittest
+import unittest2
 
 import rsa
 from rsa import bigfile, varblock, pkcs1
 
-class BigfileTest(unittest.TestCase):
+class BigfileTest(unittest2.TestCase):
 
     def test_encrypt_decrypt_bigfile(self):