Update and review Chapter 13

[Xronophobe]

I couldn't find any outdated methods or practices in the chapter. The use of $ python ... and $ python3 ... are a little inconsistent. Also, in earlier chapters I think we were being more specific about which test to run even if it was clear from the output attached below the text. In this chapter we often don't do this, rather just say "the test". On the other hand, it can be expected from the reader to get a feel for what the book is aiming to teach by now, which would mean fewer very specific step-by-step instructions, so I don't have a strong opinion on this one.

Other than these, I found it easy to read and satisfying to see how the code is becoming more concise and pretty.

[hjwp]

thanks very much Csanad, great suggestions.