Make tool python3 compatible & remove superfluous semicolon

[thierer]

With python2 deprecated, make the scripts compatible with python3, while still retaining python compatibility.

Also removes an unnecessary semicolon I found in the process.

I've confirmed with both python 2 and 3 and using 3 different disc images that the output of the new version (both on stdout and to the image file) is byte-equivalent to the python2-only version.

[ricardoquesada]

+1 for applying this patch