Support Python cells

[MichaelHatherly]

Since we now support {r} cells in notebooks via RCall.jl it does make sense to support {python} as well, likely via PythonCall.jl. Not currently high priority, but lets keep this issue to track it and potentially see whether there is much interest in the feature (+1 this issue if you'd like to see support for this).

[evetion]

I would love to see this, it would enable educational notebooks in which you show the snippets for Julia, Python and R side by side.