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/db/__init__.py

@@ -0,0 +1 @@
+"""Test the ORM layer."""