package/python3-cryptography: add python3 host variant

Add a python3 host variant since another downstream OSS component
(OP-TEE) uses buildroot and it will depend on a python3 host variant
of python-cryptography.

Signed-off-by: Donald Chan <hoiho@lab126.com>
[yann.morin.1998@free.fr:
  - drop target _DEPENDENCIES since this is a host-only package
  - instead, add host-openssl to dependencies
  - add CPE variables
  - also add sync comment for python-pip
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>