Use extras from SQLAlchemy

This avoids us declaring these ourselves. Note that this changes us from using psycopg2 to psycopg (which in reality is v3). Given we do not call this library directly, this should have no impact. It also bumps the minimum version of both SQLAlchemy and alembic, but both minimum are not super recent and are likely far more realistic lower constraints than the ancient versions we were using previously.