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/artifacts_unittest.py b/api/controller/artifacts_unittest.py
index 0a063a5..9c2e843 100644
--- a/api/controller/artifacts_unittest.py
+++ b/api/controller/artifacts_unittest.py
@@ -9,8 +9,6 @@
 
 import os
 
-import mock
-
 from chromite.api import api_config
 from chromite.api.controller import artifacts
 from chromite.api.gen.chromite.api import artifacts_pb2
@@ -24,6 +22,7 @@
 from chromite.lib import osutils
 from chromite.lib import sysroot_lib
 from chromite.service import artifacts as artifacts_svc
+from chromite.third_party import mock
 
 
 class BundleRequestMixin(object):