(not the same as 1020340)
python3-paramiko 2.4.2 is broken by python3-cryptography 2.8, which has changed
its interface.  This, in its turn, breaks duplicity.

Manually installing the newest paramiko (3.7.2) solves the problem.

/usr/lib/python3.6/site-packages/paramiko/kex_ecdh_nist.py:111:
CryptographyDeprecationWarning: encode_point has been deprecated on
EllipticCurvePublicNumbers and will be removed in a future version. Please use
EllipticCurvePublicKey.public_bytes to obtain both compressed and uncompressed
point encoding.
  hm.add_string(self.Q_C.public_numbers().encode_point())
BackendException: ssh connection to backups@mars:22 failed: private_value must
be an integer type.