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/__init__.py

@@ -27,4 +27,11 @@ else:
 
 
 # Little Hack: "Overwrites" the config module so that the environment is already set.
-config = _config.get_config('testing' if _os.getenv('TESTING') else 'production')
+config: _config.Config = _config.get_config(
+    'testing' if _os.getenv('TESTING') else 'production',
+)
+
+
+# Import `db` down here as it depends on `config`.
+# pylint:disable=wrong-import-position
+from urban_meal_delivery import db  # noqa:E402,F401 isort:skip