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/deploy_chrome_unittest.py b/scripts/deploy_chrome_unittest.py
index 054fdaa..1e5aada 100644
--- a/scripts/deploy_chrome_unittest.py
+++ b/scripts/deploy_chrome_unittest.py
@@ -7,6 +7,7 @@
 import errno
 import os
 import time
+from unittest import mock
 
 from chromite.cli.cros import cros_chrome_sdk_unittest
 from chromite.lib import chrome_util
@@ -18,7 +19,6 @@
 from chromite.lib import remote_access
 from chromite.lib import remote_access_unittest
 from chromite.scripts import deploy_chrome
-from chromite.third_party import mock
 
 
 # pylint: disable=protected-access