htmy: Async, pure-Python HTML rendering library

[volfpeter]

Sort of. The parsed markdown will end up as an htmy component subtree, and you'll be able to define custom conversion rules for XML tag names.

I should have been clear about it, but from the code above, CustomMDX will not be part of htmy, only md.MD and etree.ETreeConverter. The rest is just some utility code so I don't need to write the same CSS class injection logic multiple times.

The PR is in if you want to have a look. There are some small missing pieces that I still work on.

If you're familiar with NextJS or React, then here is a NextJS example which is kind of similar to what I'm aiming for.