When using App Engine Standard with python 3.7, none of the google.xxx libraries are available. You have to build your own, or use standard Python libraries. This goes for: auth, users, images, search, mail, taskqueue, memcache, urlfetch, deferred, etc., and even the ndb datastore interface.
For datastore, you use google-cloud-datastore or some 3rd party.
For others, you use a standard Python library, e.g.: google.auth => rauth, google.appengine.api.memcache => python-memcached
Read more here: https://cloud.google.com/appengine/docs/standard/python3/python-differences
That page recommends Google Identity Platform or Firebase Authentication to do authorization.