Workaround for Python 3 with pyparsing <= 2.0.0

Turning off packrat parsing will avoid a bug in pyparsing <= 2.0.0, however it is much slower.

http://pyparsing.svn.sourceforge.net/viewvc/pyparsing/src/pyparsing_py3.py?r1=219&r2=218&pathrev=219

This is a follow on to the discussion in #1788