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/customers.py

@@ -0,0 +1,26 @@
+"""Provide the ORM's Customer model."""
+
+import sqlalchemy as sa
+from sqlalchemy import orm
+
+from urban_meal_delivery.db import meta
+
+
+class Customer(meta.Base):
+    """A Customer of the UDP."""
+
+    # pylint:disable=too-few-public-methods
+
+    __tablename__ = 'customers'
+
+    # Columns
+    id = sa.Column(sa.Integer, primary_key=True, autoincrement=False)  # noqa:WPS125
+
+    def __repr__(self) -> str:
+        """Non-literal text representation."""
+        return '<{cls}(#{customer_id})>'.format(
+            cls=self.__class__.__name__, customer_id=self.id,
+        )
+
+    # Relationships
+    orders = orm.relationship('Order', back_populates='customer')