security/py-cryptography: Fix build with libressl 2.9.1

Backport upstream pull request #4855 by Charlie Li <ml+freebsd vishwin info>

PR: 237487
Submitted by: Maciej Pasternacki <maciej pasternacki. net> (v1)
Submitted by: gahr (v2)
Reported by: Simeon Simeonov <sgs pichove org>
Obtained from: https://github.com/pyca/cryptography/pull/4855
Tested by: gahr (all USES=ssl versions), many