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

noxfile.py

@@ -249,6 +249,8 @@ def test(session):
         '--cov-branch',
         '--cov-fail-under=100',
         '--cov-report=term-missing:skip-covered',
+        '-k',
+        'not e2e',
         PYTEST_LOCATION,
     )
     session.run('pytest', '--version')