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/bisect_git_test.py b/bisect_git_test.py
index a8e50c3..fe7bd20 100644
--- a/bisect_git_test.py
+++ b/bisect_git_test.py
@@ -8,12 +8,7 @@
 import shutil
 import tempfile
 import unittest
-
-try:
-  from unittest import mock
-except ImportError:
-  # TODO(kcwu): remove once migrated to python3
-  import mock
+from unittest import mock
 
 from bisect_kit import bisector_cli
 from bisect_kit import git_util_test