Update dependency google-auth to v1.18.0 [(#4125)](GoogleCloudPlatform/python-docs-samples#4125)

Author: renovate-bot

samples/snippets/api/requirements.txt

@@ -1,3 +1,3 @@
 google-api-python-client==1.9.3
-google-auth==1.17.2
+google-auth==1.18.0
 google-auth-httplib2==0.0.3