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/__init__.py

@@ -0,0 +1,11 @@
+"""Provide the ORM models and a connection to the database."""
+
+from urban_meal_delivery.db.addresses import Address  # noqa:F401
+from urban_meal_delivery.db.cities import City  # noqa:F401
+from urban_meal_delivery.db.connection import make_engine  # noqa:F401
+from urban_meal_delivery.db.connection import make_session_factory  # noqa:F401
+from urban_meal_delivery.db.couriers import Courier  # noqa:F401
+from urban_meal_delivery.db.customers import Customer  # noqa:F401
+from urban_meal_delivery.db.meta import Base  # noqa:F401
+from urban_meal_delivery.db.orders import Order  # noqa:F401
+from urban_meal_delivery.db.restaurants import Restaurant  # noqa:F401