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/packages_unittest.py b/api/controller/packages_unittest.py
index 2cd9f84..4b7d62f 100644
--- a/api/controller/packages_unittest.py
+++ b/api/controller/packages_unittest.py
@@ -4,6 +4,8 @@
 
 """packages controller unit tests."""
 
+from unittest import mock
+
 from chromite.api.api_config import ApiConfigMixin
 from chromite.api.controller import controller_util
 from chromite.api.controller import packages as packages_controller
@@ -18,7 +20,6 @@
 from chromite.lib import uprev_lib
 from chromite.lib.parser import package_info
 from chromite.service import packages as packages_service
-from chromite.third_party import mock
 
 
 class UprevTest(cros_test_lib.MockTestCase, ApiConfigMixin):