python2 => python3 conversion

[kkovacs]

Hello Vsevolod!!

I'm using your great bash minifier regularly, but I ran into problems now that python2 is deprecated FOR REAL.

I made the changes that were needed to run on python3, and I thought I'd contribute back. There is one change I'm not completely sure about (although it work like this and all the tests ran), maybe you could review it.

Have a great day! :)

Kristof