chore(deps): update dependency mock to v4 [(#4287)](GoogleCloudPlatform/python-docs-samples#4287)

* chore(deps): update dependency mock to v4

* specify mock version for appengine python 2

Co-authored-by: Leah Cole <[email protected]>

Author: 2 people

packages/google-cloud-bigquery-datatransfer/samples/snippets/requirements-test.txt

@@ -1,2 +1,2 @@
 pytest==5.4.3
-mock==3.0.5
+mock==4.0.2