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/pushimage_unittest.py b/scripts/pushimage_unittest.py
index ba74627..3cf9c16 100644
--- a/scripts/pushimage_unittest.py
+++ b/scripts/pushimage_unittest.py
@@ -6,6 +6,7 @@
 
 import collections
 import os
+from unittest import mock
 
 from chromite.lib import cros_build_lib
 from chromite.lib import cros_test_lib
@@ -15,7 +16,6 @@
 from chromite.lib import partial_mock
 from chromite.lib import signing
 from chromite.scripts import pushimage
-from chromite.third_party import mock
 
 
 # Use our local copy of insns for testing as the main one is not available in