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/_config.py

@@ -1,95 +0,0 @@
-"""Provide package-wide configuration.
-
-This module is "protected" so that it is only used
-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
-
-
-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."""
-
-    # 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'
-
-    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 get_config(env: str = 'production') -> Config:
-    """Get the configuration for the package.
-
-    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