Add support for Python Markdown SuperFences

[maxb2]

This supports the brace format in superfences. The blocks are processed the same as normal markdown code blocks. The only difference is the before line.

TODO

• support more language classes (I only see .python and .py3 in the superfences docs, but there is probably more.)

[adamchainz]

So SuperFences is an extension for Python Markdown? I feel like it’s ma bit niche to add support for at this time, and more importantly commit to maintaining.

You’re free to run your fork, of course, and if others chime in saying they use this then I may be more swayed.

[maxb2]

That's a fair point that it's niche.

I'll share a bit why they are useful. I'm using superfences in my own documentation as a way to pass metadata about the code block. I run the code in my markdown docs as tests (inspired by koaning/mktestdocs). The extra metadata helps control the test environment.

None of that is relevant for this project, except that I want to keep my docs formatted and need to support superfences. I thought I'd just share my work. I'll get this branch working for my own sake, but feel free to merge if you ever want to support this!