fix: make google-cloud-monitoring tests work without mock (#12317)

https://pypi.org/project/mock/ library is not needed anymore on modern
Python installation, since it is now part of the Python standard
library, available as `unittest.mock`.

Reuse approach used in generated `tests/unit/gapic/monitoring_v3/` for
manually written `tests/unit/test_query.py`.

This issue and solution was found as a part of packaging the library for
Fedora: https://bugzilla.redhat.com/show_bug.cgi?id=2263960.

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Fixes #12316 🦕

packages/google-cloud-monitoring/tests/unit/test_query.py

@@ -16,8 +16,7 @@
 
 import datetime
 import unittest
-
-import mock
+from unittest import mock
 
 from google.cloud import monitoring_v3 as monitoring_v3
 from google.cloud.monitoring_v3 import MetricServiceClient