Warning handled for Python3: DeprecationWarning: tostring()

It seems that 2to3 script does not handle the "tostring" methods very well. So I manually wrap the code to let Python3 use "tobytes" by default.

This will get rid of the annoying deprecation warnning each time tostring() is called. When I generate hundreds of images, the warnings just flush everything on screen.

I am new to github, and this is my first time using pull request. Please let me know if I can do it in a better way. pypng is a wonderful library, and I am willing to help.