Add an ORM layer

- use SQLAlchemy (and PostgreSQL) to model the ORM layer
- add the following models:
  + Address => modelling all kinds of addresses
  + City => model the three target cities
  + Courier => model the UDP's couriers
  + Customer => model the UDP's customers
  + Order => model the orders received by the UDP
  + Restaurant => model the restaurants active on the UDP
- so far, the emphasis lies on expression the Foreign Key
  and Check Constraints that are used to validate the assumptions
  inherent to the cleanded data
- provide database-independent unit tests with 100% coverage
- provide additional integration tests ("e2e") that commit data to
  a PostgreSQL instance to validate that the constraints work
- adapt linting rules a bit

src/urban_meal_delivery/db/connection.py

@@ -0,0 +1,17 @@
+"""Provide connection utils for the ORM layer."""
+
+import sqlalchemy as sa
+from sqlalchemy import engine
+from sqlalchemy import orm
+
+import urban_meal_delivery
+
+
+def make_engine() -> engine.Engine:  # pragma: no cover
+    """Provide a configured Engine object."""
+    return sa.create_engine(urban_meal_delivery.config.DATABASE_URI)
+
+
+def make_session_factory() -> orm.Session:  # pragma: no cover
+    """Provide a configured Session factory."""
+    return orm.sessionmaker(bind=make_engine())