.......I tried an experiment with a remote server that I control and pysftp works perfectly there. A difference that I know of is that this server is using ubuntu 18.04 and we don't use passwords, but a private_key. Also this server is using the openssh internal sftp.
I believe the failing server is using an earlier version of openssh or OS as it wants to use ssh-dss which is now considered unsafe (I believe).
-- Robin Becker -- https://mail.python.org/mailman/listinfo/python-list