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

* chore(deps): update dependency mock to v4

* specify mock version for appengine python 2

Co-authored-by: Leah Cole <d7cd7ba333275e71ca359a2e20dc85dbf2c605f7@google.com>
googleapis/python-bigquery-datatransfer,googleapis/python-bigquery-datatransfer