setup.md: Update Python installation instructions

[vinisalazar]

Hi,

Please see #501 for discussion.

If merged, this PR will:

• Add the _includes/python_install.html file with setup instructions, the same present in carpentries/workshop-template repository. This file contains embedded video instructions for each operating system.
• Include said file in the setup.md (and delete previous instructions). I also moved the "Getting the Data Section" up so it does not stay hidden below the Python setup section.
• Update the assets/css/lesson.scss file according to the carpentries/workshop-template repository.

I do know that altering static files may be a bit risky, but that was needed so the Python setup instructions block would look right. I rendered the lesson locally on OS X and could not find any problems.

Best, V

[vinisalazar]

Pinging @alee, could we get a position from the maintainers regarding this one?

Thank you for any assistance you can provide, Vini

[vahtras]

Looks good to me!