package/python3-cffi: add python3 host variant

Add a python3 host variant since we are adding a python3 host variant of
python-cryptography and it is dependent on this.

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