Add database migrations

- use Alembic to migrate the PostgreSQL database
  + create initial migration script to set up the database,
    as an alternative to db.Base.metadata.create_all()
  + integrate Alembic into the test suite; the db_engine fixture
    now has two modes:
    * create the latest version of tables all at once
    * invoke `alembic upgrade head`
    => the "e2e" tests are all run twice, once in each mode; this
       ensures that the migration scripts re-create the same database
       schema as db.Base.metadata.create_all() would
    * in both modes, a temporary PostgreSQL schema is used to create the
      tables in
    => could now run "e2e" tests against production database and still
       have isolation
- make the configuration module public (to be used by Alembic)
- adjust linting rules for Alembic

src/urban_meal_delivery/configuration.py

@@ -0,0 +1,101 @@
+"""Provide package-wide configuration.
+
+This module provides utils to create new `Config` objects
+on the fly, mainly for testing and migrating!
+
+Within this package, use the `config` proxy at the package's top level
+to access the current configuration!
+"""
+
+import datetime
+import os
+import random
+import string
+import warnings
+
+import dotenv
+
+
+dotenv.load_dotenv()
+
+
+def random_schema_name() -> str:
+    """Generate a random PostgreSQL schema name for testing."""
+    return 'temp_{name}'.format(
+        name=''.join(
+            (random.choice(string.ascii_lowercase) for _ in range(10)),  # noqa:S311
+        ),
+    )
+
+
+class Config:
+    """Configuration that applies in all situations."""
+
+    # pylint:disable=too-few-public-methods
+
+    CUTOFF_DAY = datetime.datetime(2017, 2, 1)
+
+    # If a scheduled pre-order is made within this
+    # time horizon, we treat it as an ad-hoc order.
+    QUASI_AD_HOC_LIMIT = datetime.timedelta(minutes=45)
+
+    DATABASE_URI = os.getenv('DATABASE_URI')
+
+    # The PostgreSQL schema that holds the tables with the original data.
+    ORIGINAL_SCHEMA = os.getenv('ORIGINAL_SCHEMA') or 'public'
+
+    # The PostgreSQL schema that holds the tables with the cleaned data.
+    CLEAN_SCHEMA = os.getenv('CLEAN_SCHEMA') or 'clean'
+
+    ALEMBIC_TABLE = 'alembic_version'
+    ALEMBIC_TABLE_SCHEMA = 'public'
+
+    def __repr__(self) -> str:
+        """Non-literal text representation."""
+        return '<configuration>'
+
+
+class ProductionConfig(Config):
+    """Configuration for the real dataset."""
+
+    # pylint:disable=too-few-public-methods
+
+    TESTING = False
+
+
+class TestingConfig(Config):
+    """Configuration for the test suite."""
+
+    # pylint:disable=too-few-public-methods
+
+    TESTING = True
+
+    DATABASE_URI = os.getenv('DATABASE_URI_TESTING') or Config.DATABASE_URI
+    CLEAN_SCHEMA = os.getenv('CLEAN_SCHEMA_TESTING') or random_schema_name()
+
+
+def make_config(env: str = 'production') -> Config:
+    """Create a new `Config` object.
+
+    Args:
+        env: either 'production' or 'testing'; defaults to the first
+
+    Returns:
+        config: a namespace with all configurations
+
+    Raises:
+        ValueError: if `env` is not as specified
+    """  # noqa:DAR203
+    config: Config
+    if env.strip().lower() == 'production':
+        config = ProductionConfig()
+    elif env.strip().lower() == 'testing':
+        config = TestingConfig()
+    else:
+        raise ValueError("Must be either 'production' or 'testing'")
+
+    # Without a PostgreSQL database the package cannot work.
+    if config.DATABASE_URI is None:
+        warnings.warn('Bad configurartion: no DATABASE_URI set in the environment')
+
+    return config