databases/py-alembic16: New port

Alembic is a new database migrations tool, written by the author of
SQLAlchemy. 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.

WWW: http://alembic.zzzcomputing.com/

PR: 258941
Reported by: einar@isnic.is