switch to unittest.mock

We've patched Python 3.6 inside the SDK to include the mock fix we need.

BUG=b:187789661
TEST=CQ passes

Cq-Depend: chromium:2911618
Change-Id: Ib6ba4f9176cf8e121220a3bfe5015dcb5e3d7499
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2910244
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/api/controller/binhost_unittest.py b/api/controller/binhost_unittest.py
index fa45e5a..2ca8377 100644
--- a/api/controller/binhost_unittest.py
+++ b/api/controller/binhost_unittest.py
@@ -5,6 +5,7 @@
 """Unittests for Binhost operations."""
 
 import os
+from unittest import mock
 
 from chromite.api import api_config
 from chromite.api.controller import binhost
@@ -13,7 +14,6 @@
 from chromite.lib import cros_test_lib
 from chromite.lib import osutils
 from chromite.service import binhost as binhost_service
-from chromite.third_party import mock
 
 
 class GetBinhostsTest(cros_test_lib.MockTestCase, api_config.ApiConfigMixin):