Some code in the guide is still Python 2

[fjhheras]

For example, Python 3 would fail with the current print statements and xrange in exercises 3.9 and 3.11.

[ramon-astudillo]

thanks! can you do a PR?

[fjhheras]

Yes, I can. Do you want it to be merged before the beginning of the course? Otherwise I can wait in case I see other instances of the problem elsewhere in the guide and solve all of them at once

EDIT: I did (see #144 ). Please feel free to merge, or otherwise I will keep pushing other examples of Python 2 to the same branch that I or other people identify.