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/_config.py

@@ -6,9 +6,10 @@ via the `config` proxy at the package's top level.
 That already loads the correct configuration
 depending on the current environment.
 """
-
 import datetime
 import os
+import random
+import string
 import warnings
 
 import dotenv
@@ -17,6 +18,13 @@ import dotenv
 dotenv.load_dotenv()
 
 
+def random_schema_name() -> str:
+    """Generate a random PostgreSQL schema name for testing."""
+    return ''.join(
+        random.choice(string.ascii_lowercase) for _ in range(10)  # noqa:S311
+    )
+
+
 class Config:
     """Configuration that applies in all situations."""
 
@@ -57,7 +65,7 @@ class TestingConfig(Config):
     TESTING = True
 
     DATABASE_URI = os.getenv('DATABASE_URI_TESTING') or Config.DATABASE_URI
-    CLEAN_SCHEMA = os.getenv('CLEAN_SCHEMA_TESTING') or Config.CLEAN_SCHEMA
+    CLEAN_SCHEMA = os.getenv('CLEAN_SCHEMA_TESTING') or random_schema_name()
 
 
 def get_config(env: str = 'production') -> Config: