finish Py2 to 3 transition

[a-detiste]

:-)

[dragotin]

Thank you very much! If you find time, there is another utility called https://github.com/dragotin/kraft/blob/master/tools/watermarkpdf.py that would deserve a review. My python knowledge is limited...

[a-detiste]

I grepped through the whole project and found nothing else to fix.