import pysftp

pub_key = 'key_file.pub'

cnopts = pysftp.CnOpts()
cnopts.hostkeys.load(pub_key)

server = pysftp.Connection(host=SFTP_SITE, username=DOWNLOAD['USERNAME'],
                            password=DOWNLOAD['PASSWORD'], cnopts=cnopts, port=SFTP_PORT)

sftp-test.example.com ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAIE...

SSHException: Unable to connect to sftp-test-gwta.rbcis.com: [WinError 10060]
A connection attempt failed because the connected party did not properly respond
after a period of time, or established connection failed
because connected host has failed to respond.