import pysftp
import paramiko
from paramiko.py3compat import decodebytes

keydata = b"""4049a539f13cba06a874c4b6aadd0cc4"""

key = paramiko.RSAKey(data=decodebytes(keydata))
cnopts = pysftp.CnOpts()
cnopts.hostkeys.add('sftp://adwords-flights.almundo.it', 'ssh-rsa', key)

with pysftp.Connection('adwords-flights.almundo.it', username='google', password='Vah0oozu', cnopts=cnopts) as sftp:
    sftp.get('remote_file')