Require the SQLAlchemy URI on the database model (#8720)

* Require the SQLAlchemy URI when creating a database

* Add migration to make dbs.sqlalchemy_uri not-nullable

* Fixes for black, isort, tests

* Alter migration to use current revision from master as downgrade target

* Update tests to support new db constraint

* black

tests/base_tests.py

@@ -228,6 +228,7 @@ class SupersetTestCase(TestCase):
             cls=models.Database,
             criteria={"database_name": database_name},
             session=db.session,
+            sqlalchemy_uri="sqlite://test",
             id=db_id,
             extra=extra,
         )