Mediagoblin: Python 3 port + switch to Alembic

by Boris Bobrov for GNU Project

Port all of MediaGoblin's code to python 3: https://issues.mediagoblin.org/ticket/813 Currently MediaGoblin uses sqlalchemy-migrate, but it's fairly outdated and unmaintained. Port the migration infrastructure to using Alembic instead. Which is also python3-compatible.