import mock from third_party explicitly

Stop relying on third_party/ leaking into sys.path all the time.

BUG=b:187794934
TEST=CQ passes

Change-Id: If233859063e6d2a1f2fdaa851febcbe8f536896a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2896053
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/api/controller/image_unittest.py b/api/controller/image_unittest.py
index c808c17..fbe01df 100644
--- a/api/controller/image_unittest.py
+++ b/api/controller/image_unittest.py
@@ -9,8 +9,6 @@
 
 import os
 
-import mock
-
 from chromite.api import api_config
 from chromite.api import controller
 from chromite.api.controller import image as image_controller
@@ -24,6 +22,7 @@
 from chromite.lib import osutils
 from chromite.scripts import pushimage
 from chromite.service import image as image_service
+from chromite.third_party import mock
 
 
 class CreateTest(cros_test_lib.MockTempDirTestCase, api_config.ApiConfigMixin):