Don't add from __future__ import print_function in generated file

[santazhang]

from __future__ import print_function must be placed at beginning of python file. But the user might have a header code section specified in .g file (before %% marker), and in this case the generated parser will not work. I think it's better to let users add from __future__ import print_function themselves.

[felixp7]

I just ran into the same issue, and worked around it by editing the generated parser. The simpler solution is to insert the pre-parse code from the grammar file after the import statements, rather than before. Nothing else needs to be changed.

[jepler]

#6 resolves this in a way that still allows the from __future__ import print_function to be in the generated file, similar to what @felixplesoianu described doing manually.