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/sdk_unittest.py b/api/controller/sdk_unittest.py
index 43a9823..9eebbd6 100644
--- a/api/controller/sdk_unittest.py
+++ b/api/controller/sdk_unittest.py
@@ -4,13 +4,14 @@
 
 """SDK tests."""
 
+from unittest import mock
+
 from chromite.api import api_config
 from chromite.api.controller import sdk as sdk_controller
 from chromite.api.gen.chromite.api import sdk_pb2
 from chromite.lib import cros_build_lib
 from chromite.lib import cros_test_lib
 from chromite.service import sdk as sdk_service
-from chromite.third_party import mock
 
 
 class SdkCreateTest(cros_test_lib.MockTestCase, api_config.ApiConfigMixin):