chore(deps): update dependency mock to v4.0.3 (#327)

Author: renovate-bot

packages/google-cloud-spanner/samples/samples/requirements-test.txt

@@ -1,3 +1,3 @@
 pytest==6.2.4
-mock==4.0.2
+mock==4.0.3
 google-cloud-testutils==0.2.0