sqlalchemy 2.0 breaking db migration scripts

[Romain Dupont]

INFO [alembic.runtime.migration] Running upgrade 019 -> 020, add type to task execution
[...]
TypeError: begin() got an unexpected keyword argument 'subtransactions'

https://github.com/openstack/mistral/blob/master/mistral/db/sqlalchemy/migration/alembic_migrations/versions/020_add_type_to_task_execution.py#L60

https://pypi.org/project/SQLAlchemy/2.0.0/#history 2.0.0 released January 26

https://docs.sqlalchemy.org/en/14/orm/session_api.html#sqlalchemy.orm.Session.params.future

"The “subtransactions” feature of Session.begin() is removed in version 2.0 and is disabled when the future flag is set."