[SCons] Fix to skip Python startup messages

This fixes an issue when the Python interpreter prints startup messages that
appear before the expected output lines. Known to be a problem with the Anaconda
Python distribution.

git-svn-id: https://cantera.googlecode.com/svn/cantera/branches/2.1@2519 02a645c2-efd0-11dd-984d-ab748d24aa7e