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/__init__.py

@@ -9,7 +9,7 @@ Example:
 import os as _os
 from importlib import metadata as _metadata
 
-from urban_meal_delivery import _config  # noqa:WPS450
+from urban_meal_delivery import configuration as _configuration
 
 
 try:
@@ -26,8 +26,8 @@ else:
     __version__ = _pkg_info['version']
 
 
-# Little Hack: "Overwrites" the config module so that the environment is already set.
-config: _config.Config = _config.get_config(
+# Global `config` object to be used in the package.
+config: _configuration.Config = _configuration.make_config(
     'testing' if _os.getenv('TESTING') else 'production',
 )
 

src/urban_meal_delivery/configuration.py

@@ -1,11 +1,12 @@
 """Provide package-wide configuration.
 
-This module is "protected" so that it is only used
-via the `config` proxy at the package's top level.
+This module provides utils to create new `Config` objects
+on the fly, mainly for testing and migrating!
 
-That already loads the correct configuration
-depending on the current environment.
+Within this package, use the `config` proxy at the package's top level
+to access the current configuration!
 """
+
 import datetime
 import os
 import random
@@ -20,8 +21,10 @@ 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
+    return 'temp_{name}'.format(
+        name=''.join(
+            (random.choice(string.ascii_lowercase) for _ in range(10)),  # noqa:S311
+        ),
     )
 
 
@@ -44,6 +47,9 @@ class Config:
     # 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>'
@@ -68,8 +74,8 @@ class TestingConfig(Config):
     CLEAN_SCHEMA = os.getenv('CLEAN_SCHEMA_TESTING') or random_schema_name()
 
 
-def get_config(env: str = 'production') -> Config:
-    """Get the configuration for the package.
+def make_config(env: str = 'production') -> Config:
+    """Create a new `Config` object.
 
     Args:
         env: either 'production' or 'testing'; defaults to the first