Fix OpenStack StyleGuide rule H216 (On by default in latest version) (#7535)

ref: https://opendev.org/openstack/hacking/commit/b921c4de513c9cc624d6ecf68e4f4493e6e72c0d

contrib/inventory_builder/tests/test_inventory.py

@@ -13,8 +13,8 @@
 # under the License.
 
 import inventory
-import mock
 import unittest
+from unittest import mock
 
 from collections import OrderedDict
 import sys