security/py-cryptography: support LibreSSL 3.4.0
Merged upstream as https://github.com/pyca/cryptography/pull/6360
and backported to this version.
While here, remove remaining FreeBSD 11 cruft
Approved by: fluffy (mentor), koobs (implicit: MAINTAINER_POLICY)
Differential Revision: https://reviews.freebsd.org/D32281
(cherry picked from commit 5b57210d0d0a7d74c9f8b4895907b34f2f34473d)