Upgrade sqlalchemy

Adapt code to prevent new warnings and errors (as of SQLAlchemy 1.4):
- Overlapping foreign key columns could be set in a conflicting way
  => This is prevented by the application logic
  => Ignore the warning by setting a `overlaps` flag
- Transaction already rolled back
  => This only happens when tests cause an `IntegrityError` on purpose
  => Filter away the corresponding warning in the fixture
- Query returns `Row` objects and not scalars
  => Add genexpr to pull out `primary_id`

src/urban_meal_delivery/db/addresses_addresses.py

@@ -89,13 +89,12 @@ class DistanceMatrix(meta.Base):
     # Relationships
     first_address = orm.relationship(
         'Address',
-        back_populates='_distances1',
         foreign_keys='[DistanceMatrix.first_address_id, DistanceMatrix.city_id]',
     )
     second_address = orm.relationship(
         'Address',
-        back_populates='_distances2',
         foreign_keys='[DistanceMatrix.second_address_id, DistanceMatrix.city_id]',
+        overlaps='first_address',
     )
 
     @classmethod