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

tests/test_config.py

@@ -43,3 +43,11 @@ def test_no_database_uri_set(env, monkeypatch):
 
     with pytest.warns(UserWarning, match='no DATABASE_URI'):
         config_mod.get_config(env)
+
+
+def test_random_testing_schema():
+    """CLEAN_SCHEMA is randomized if not seti explicitly."""
+    result = config_mod.random_schema_name()
+
+    assert isinstance(result, str)
+    assert len(result) <= 10