[Python] Fix to conversion of samples from Python 3 to Python 2

Some calls to the print function weren't being converted to print statements
correctly by 3to2, so now we use the alternative fix, which is to use the
backported print function.

Fixes Issue 216.

Cherry-pick of trunk r2952.

interfaces/cython/SConscript

@@ -208,6 +208,7 @@ if localenv['python_package'] == 'new':
         def convert_example(target, source, env):
             shutil.copyfile(source[0].abspath, target[0].abspath)
             subprocess.call(['3to2', '--no-diff', '-n', '-w','-x', 'str',
+                             '-f', 'all', '-f', 'printfunction', '-x', 'print',
                              '-x', 'open', target[0].abspath])
 
         for subdir in subdirs('cantera/examples'):