pmt_to_python.py Python 2.6 compatibility

Should fix http://lists.gnu.org/archive/html/discuss-gnuradio/2014-04/msg00042.html

Tested on a CentOS 6.5 VM.

Added a little, really minimal, QA during development of the fix and decided to keep it. 
pmt_to_python definitively needs more QA code, though.