security/py-paramiko: Backport upstream fix on CryptographyDeprecationWarnings

Following cryptography 2.5 release the py-paramiko generates
the deprecation warnings that breaks user-cases relying on its output.

The issue has been tracked and fixed upstream, this patch backports on our tree,
until upstream's next release.

Reported by: filis
Reviewed by: sbz (maintainer)
Approved by: sbz (maintainer)
Sponsored by: Platform.sh
Differential Revision: https://reviews.freebsd.org/D20384