gitstats: make it work with Python3

[bazsi]

It's basically created using 2to3 and adding one decode call. I am not sure it works perfectly but it did generate output
for my repository.

[bazsi]

Same as #105 (should have checked first)

[shenxianpeng]

Hi! I’ve revived gitstats with Python 3 support in the repository: https://github.com/shenxianpeng/gitstats. Feel free to check it out if needed.