Support for python console (`pycon`) as in doctests

[staadecker]

Here's an excerpt from some documentation I'm currently writing:

m.Buy = pf.Variable(data[["food"]], lb=0)

If you print the variable, you'll see it actually contains a tofu and chickpeas variable!

>>> m.Buy
<Variable name=Buy lb=0 size=2 dimensions={'food': 2}>
[tofu]: Buy[tofu]
[chickpeas]: Buy[chickpeas]

Currently the pycon continuation code fence isn't detected as a test. It would be great if it could be and if it could run in the same way that doctests run!

[freider]

I think this has come up before. Not sure how hard it would be to integrate doctests and get it to play nice with our fixture support etc., but I'm not against it in principle so happy to look at a contribution if you want to make a PR :)