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

migrations/env.py

@@ -0,0 +1,45 @@
+"""Configure Alembic's migration environment."""
+
+import os
+from logging import config as log_config
+
+import sqlalchemy as sa
+from alembic import context
+
+from urban_meal_delivery import config as umd_config
+from urban_meal_delivery import db
+
+
+# Disable the --sql option, a.k.a, the "offline mode".
+if context.is_offline_mode():
+    raise NotImplementedError('The --sql option is not implemented in this project')
+
+
+# Set up the default Python logger from the alembic.ini file.
+log_config.fileConfig(context.config.config_file_name)
+
+
+def include_object(obj, _name, type_, _reflected, _compare_to):
+    """Only include the clean schema into --autogenerate migrations."""
+    if type_ in {'table', 'column'} and obj.schema != umd_config.DATABASE_SCHEMA:
+        return False
+
+    return True
+
+
+engine = sa.create_engine(umd_config.DATABASE_URI)
+
+with engine.connect() as connection:
+    context.configure(
+        connection=connection,
+        include_object=include_object,
+        target_metadata=db.Base.metadata,
+        version_table='{alembic_table}{test_schema}'.format(
+            alembic_table=umd_config.ALEMBIC_TABLE,
+            test_schema=(f'_{umd_config.CLEAN_SCHEMA}' if os.getenv('TESTING') else ''),
+        ),
+        version_table_schema=umd_config.ALEMBIC_TABLE_SCHEMA,
+    )
+
+    with context.begin_transaction():
+        context.run_migrations()