Make metaclass compatible for both 2 and 3
Make metaclass compatible for both 2 and 3

fsys/alembic,6si/alembic,fsys/alembic,Frozenball/alembic,6si/alembic,chishaku/alembic,graingert/alembic,zzzeek/alembic,ideadevice/alembic,moonrabbit/alembic,ideadevice/alembic,moonrabbit/alembic,Frozenball/alembic,sqlalchemy/alembic,ImaginationForPeople/alembic