Alembic is a database migrations tool written by the author of
SQLAlchemy. A migrations tool can emit ALTER statements to a database
in order to change the structure of tables and other constructs,
provides a system whereby "migration scripts" may be constructed; each
script indicates a particular series of steps that can "upgrade" a
target database to a new version, and optionally a series of steps
that can "downgrade" similarly, doing the same steps in reverse, and
allows the scripts to execute in some sequential manner.