Евгений

...
cnopts = pysftp.CnOpts()
    cnopts.hostkeys = None  # disable host key checking.

    # Connection to external SFTP server
    with pysftp.Connection(host=SFTP_HOST,
                           port=SFTP_PORT,
                           username=SFTP_USER,
                           password=fernet_decrypt(fernet_key=SFTP_KEY, encoded=SFTP_PASS_ENC),
                           cnopts=cnopts,
                           ) as sftp:

        # Change external directory
        with sftp.cd(EXTERNAL_DIR):
            external_dir = sftp.listdir()
...