[CI] Add configuration for PostgreSQL in CircleCI

[wo0dyn]

It would be nice to add configuration for PostgreSQL in CircleCI settings so we can check that SQL performances are OK against a PostgreSQL database as well.

Ideally we can keep both SQLite and PostgreSQL.

[brunobord]

found this: https://discuss.circleci.com/t/django-postgresql-and-circleci/15032/3

[brunobord]

and... a couple of years after... ;o)

[wo0dyn]

It's never too late. 😉