Comment 4 for bug 1307266

Seems to be, that you don't use monkey-patching for sqlalchemy-migrate in your code. See https://github.com/openstack/nova/blob/master/nova/openstack/common/db/sqlalchemy/migration.py#L131.
This monkey-patch adds a workaround for SQLite's inability to alter things and a possibility to use the unified drop_unique_constraint()
This feature was added to Oslo in patch https://review.openstack.org/#/c/31016/ (Add a monkey-patching util for sqlalchemy-migrate)