scripts: use unittest.mock

Use mock from Python 3's stdlib rather than external 3rd party package.

BUG=chromium:1027199
TEST=unittests pass

Change-Id: I1528e2e1740f94a00375790422b2571135e19912
Reviewed-on: https://chromium-review.googlesource.com/c/aosp/platform/system/update_engine/+/2690715
Reviewed-by: Vyshu Khota <vyshu@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/payload_info_unittest.py b/scripts/payload_info_unittest.py
index 07bb679..3d0734b 100755
--- a/scripts/payload_info_unittest.py
+++ b/scripts/payload_info_unittest.py
@@ -25,13 +25,12 @@
 
 import sys
 import unittest
+from unittest import mock
 
 from contextlib import contextmanager
 
 from six.moves import StringIO
 
-import mock  # pylint: disable=import-error
-
 import payload_info
 import update_payload
 
diff --git a/scripts/update_payload/checker_unittest.py b/scripts/update_payload/checker_unittest.py
index 993b785..be17a57 100755
--- a/scripts/update_payload/checker_unittest.py
+++ b/scripts/update_payload/checker_unittest.py
@@ -29,11 +29,10 @@
 import itertools
 import os
 import unittest
+from unittest import mock  # pylint: disable=import-error
 
 from six.moves import zip
 
-import mock  # pylint: disable=import-error
-
 from update_payload import checker
 from update_payload import common
 from update_payload import test_utils