Psycopg2 - PGBouncer - Postgresql > Server does not support SSL but SSL was required
The reason that this command worked:

psql --set=sslmode=verify-full -h DBHOST -p DBPORT -U USERNAME DBNAME


Is that --set just creates a user-defined variable inside the psql program with the name of 'sslmode'.  That name is not special to psql, it does nothing with your connection options and you just connect without ssl.