Here's how you configure your App Engine standard environment project to use Cloud SQL: https://cloud.google.com/appengine/docs/standard/python3/usi...
The Cloud SQL for PostgreSQL docs contain code samples to get you started: https://cloud.google.com/sql/docs/postgres/
This should all work with SQLAlchemy.
https://cloud.google.com/sql/docs/postgres/connect-app-engin...
Note that only node and Java are listed under non-Flex.
I got burned by this and had to start over with MySQL.
It is possible on the new (beta) Python 3. 7 runtime on AppEngine standard:
https://cloud.google.com/appengine/docs/standard/python3/usi...
If you try Cloud SQL and find that it doesn't work with PostgreSQL on this new runtime, that's a bug and we need to fix it.