MFH: r476488 security/py-cryptography: Update to 2.3

This version update fixes builds with libressl >= 2.7.

Builds with openssl-devel are broken due to an underlying lang/python* port
issue [1].

Changelog:

https://github.com/pyca/cryptography/blob/2.3/CHANGELOG.rst

[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=229223

PR: 226906
Submitted by: Charli Li <ml+freebsd vishwin info>
Reported by: many
Security: 9e2d0dcf-9926-11e8-a92d-0050562a4d7b

Approved by: ports-secteam (miwi)