It was moved to __init__ to avoid recalculating, but the directory could have changed, so we need to wait to do the work. Instead, only do the relpath on systems that need it (Windows Py 2).
Since the self.filename attribute doesn't change during the lifetime of a SqliteDb object, we can move its relpath transformation in the init method, instead of doing it every time _connect is called, resulting in a ~30% performances gain.