Get the db engine directly from the extensions dictionary.

Resolves deprecation of the .db attribute in Flask-SQLAlchemy 3.0.0

src/flask_marshmallow/__init__.py

@@ -111,7 +111,7 @@ def init_app(self, app):
 
         # If using Flask-SQLAlchemy, attach db.session to SQLAlchemySchema
         if has_sqla and "sqlalchemy" in app.extensions:
-            db = app.extensions["sqlalchemy"].db
+            db = app.extensions["sqlalchemy"]
             if self.SQLAlchemySchema:
                 self.SQLAlchemySchema.OPTIONS_CLASS.session = db.session
             if self.SQLAlchemyAutoSchema: