This keeps references to sftp only in very basic internal calls that can be overriden.

SSH only client copies files with cat, triggering full file content to be printed here. Which can be a lot.

It's a variant of the ssh transport, inheriting it and replacing each sftp command by an equivalent pure ssh call.
This has been (quickly) tested to work to submit jobs and gather results on two different systems without sftp access (slurm and direct).