Support Python excercises

[kernc]

Hi,

It would benefit the teaching community at large if GitbookIO's excercise blocks could be made to support Python (3+), leveraging one of the existing Python-to-JavaScript solutions (e.g Brython, which supports also Python3, looks easy to use, is well documented and with an active community, or perhaps Skulpt, but the pioneers seem pretty firm on Python2.6 😟).

As I observe, this plugin-excercises module looks quite extensible, but as I'm not that comfortable in JavaScript, could you please guide me a bit, what would it take for it to support

```py

code blocks, wrapping them somehow in a brython sandbox, so I can test it all for you and submit a PR?