unittests: switch to Python 3 stdlib mock

We don't want to use the old external 3rd party module anymore,
so switch to the stdlib version in Python 3 itself.

BUG=chromium:1015720
TEST=unittests pass

Change-Id: I3fcdded0cbd9a89f3793928d5295f75f9b804e9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/repohooks/+/2370107
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/pre-upload_unittest.py b/pre-upload_unittest.py
index 0702f54..a318242 100755
--- a/pre-upload_unittest.py
+++ b/pre-upload_unittest.py
@@ -12,8 +12,7 @@
 import datetime
 import os
 import sys
-
-import mock
+from unittest import mock
 
 import errors