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/scripts/repo_sync_manifest_unittest.py b/scripts/repo_sync_manifest_unittest.py
index bae7f3a..6cc0e61 100644
--- a/scripts/repo_sync_manifest_unittest.py
+++ b/scripts/repo_sync_manifest_unittest.py
@@ -5,12 +5,12 @@
 """Tests for scripts/repo_sync_manifest."""
 
 import os
+from unittest import mock
 
 from chromite.cbuildbot import manifest_version
 from chromite.cbuildbot import repository
 from chromite.lib import cros_test_lib
 from chromite.scripts import repo_sync_manifest
-from chromite.third_party import mock
 
 
 class RepoSyncManifestTest(cros_test_lib.RunCommandTempDirTestCase):