Poudriere and python framework of ports

[Yasuhiro KIMURA]

I proposed following patch to Bug 205789:

https://bugs.freebsd.org/bugzilla/attachment.cgi?id=165035&action=diff

The intention of my patch is to make the port always use python 3
regardless of default python version setting.

On real 10.2-RELEASE environment it just works as is intended. That
is:

* "cd /usr/ports/mail/postfix-policyd-spf-python; make install"
  succeeds without any error.
* Both mail/postfix-policyd-spf-python itself and dependents (such as
  mail/py-authres or mail/py-pyspf) are build and installed as
  python 3 packages (py34-postfix-policyd-spf-python-1.3.2_2,
  py34-authres-0.800, py34-pyspf-2.0.12_3, and etc.).

But on poudriere with 10.2-R jail the result is different as
following:

* "poudriere testport -j 102release -o mail/postfix-policyd-spf-python"
  fails at check-sanity phase. 
* Some dependents (such as mail/py-authres or mail/py-pyspf) are build
  as python 2 packages (py27-authres-0.800, py27-pyspf-2.0.12_3, and
  etc.)
  
Then what is the cause of failure on poudriere? Is there something
wrong with my patch, or is this bug of either poudriere or python
framework of ports? Would someone please let me know?

Best Regards.

---
Yasuhiro KIMURA