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

pyproject.toml

@@ -27,6 +27,7 @@ repository = "https://github.com/webartifex/urban-meal-delivery"
 [tool.poetry.dependencies]
 python = "^3.8"
 
+alembic = "^1.4.2"
 click = "^7.1.2"
 psycopg2 = "^2.8.5"  # adapter for PostgreSQL
 python-dotenv = "^0.14.0"
@@ -56,6 +57,7 @@ wemake-python-styleguide = "^0.14.1"  # flake8 plug-in
 packaging = "^20.4"  # used to test the packaged version
 pytest = "^6.0.1"
 pytest-cov = "^2.10.0"
+pytest-env = "^0.6.2"
 xdoctest = { version="^0.13.0", extras=["optional"] }
 
 # Documentation