bisect-kit: clean up code for python2 compability

 - python3 added 'mock' to unittest module
 - all classes are inherits from "object" in python3

BUG=None
TEST=unittest

Change-Id: I47cc00c006cd2d81ec357a6ad5e514a15bc1f667
diff --git a/runner_test.py b/runner_test.py
index a13c530..eb09b17 100644
--- a/runner_test.py
+++ b/runner_test.py
@@ -7,12 +7,7 @@
 from __future__ import print_function
 import argparse
 import unittest
-
-try:
-  from unittest import mock
-except ImportError:
-  # TODO(kcwu): remove once migrated to python3
-  import mock
+from unittest import mock
 
 import runner
 from bisect_kit import common