testing/py3-flask-sqlalchemy: upgrade to 3.0.3

• Show helpful errors when mistakenly using multiple SQLAlchemy instances for the same app, or without calling init_app. 🇵🇷1151
• Fix issue with getting the engine associated with a model that uses polymorphic table inheritance. :issue:1155

-- https://flask-sqlalchemy.palletsprojects.com/en/3.0.x/changes/#version-3-0-3

Also re-enabled checks and applying as contributor.