python3 support

[akerl]

This is based on #90 but accounts for more recent changes

[xiaopeng163]

There are some code lines need to be fixed, like the print statement. If you just use print as a function, then the code does not support python2.

One solution is put from __future__ import print_function at the beginning of the file where we use print as a function.

the other changed parts also must consider the compatibility of python 2 and 3.

[eirnym]

@xiaopeng163 it's not true. Both python 2.7.15 and sytem python 2.7.10 will print a line with letters Hello world for an one-line program below

print("Hello world")

[xiaopeng163]

@eirnym yes, but if we want to say python 2 support, we may not only support python 2,7 +

[eirnym]

(you have no lines with syntax should be changed, so 2.6 could be used as a minimum requirement)

There's a few points to notice:

• You use a very deprecated and discoraged syntax in one place to throw an exception (just add brackets)
• Most print usages are print() already, except of 6 which easy to convert. Future print_function support is starting with Python 2.6
• you have a calls to raw_input and check if object is a unicode on the second (this easily covered by six or manually)

BTW, I don't know which python less than 2.6 you really want to support in 2018 as I don't know a system where you are not able to run at least Python 2.6.
With print() in worst case will end up with messages in brackets, like (printed message), with upgrading this line with deprecated syntax will break compatibility with python 2.3 or even less. Finally, patching will not break anything. I don't believe that this script would run on Python 2.2 or less due to dependencies you use.