security/py-pyptlib: Limit Python version to 2.7

This port is not Python 3 compatible, failing with:

File "/usr/local/lib/python3.6/site-packages/pyptlib/util/__init__.py", line 56
  m = re.match(ur'^\[(.+)\]:(\d*)$', spec)
                                  ^

SyntaxError: invalid syntax

Reviewed by: koobs, cs
Approved by: cs (maintainer), koobs (ports)
Differential Revision: https://reviews.freebsd.org/D13464