Alembic is a new database migrations tool, written by the author
of SQLAlchemy <http://www.sqlalchemy.org>. A migrations tool
offers the following functionality:
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.
Allows the scripts to execute in some sequential manner.