python3Packages.cryptography: 2.8 -> 2.9

Backwards incompatible changes:
- Support for Python 3.4 has been removed due to low usage and
  maintenance burden.
- Support for OpenSSL 1.0.1 has been removed. Users on older version of
  OpenSSL will need to upgrade.
- Support for LibreSSL 2.6.x has been removed.
- Reversed the order in which rfc4514_string() returns the RDNs as
  required by RFC 4514.

Note: The first three changes should have no impact on Nixpkgs as we
already removed Python 3.4 and OpenSSL 1.0.1. Additionally we don't
support LibreSSL for this package.