net/py-pyzmq: update to 15.2.0

Also, pyzmq now ships asyncio support that requires support of PEP 0409 [1]
and depends on asyncio. As PEP0409 is supported in Python 3.3+ and
asyncio is included into stdlib in 3.4, for 3.3 add dependency on devel/py-asyncio.

1: https://www.python.org/dev/peps/pep-0409/

Reviewed by: koobs
Differential Revision: D5531