[PATCH] docs: pin mistune version due to breakage

[Stephen Finucane]

On Mon, 2021-12-06 at 16:42 +0800, You-Sheng Yang wrote:
> mistune made a new release which breaks API referenced by Sphinx v4.3.1.
> 
> Closes: #442
> Signed-off-by: You-Sheng Yang <vicamo at gmail.com>

Reviewed-by: Stephen Finucane <stephen at that.guru>

I was hoping to be able to fix the underlying issue myself, but the changes in
mistune are rather significant. I've applied this. Thank you!

For anyone curious, this is brought in by sphinxcontrib-openapi, which in turn
includes m2r (to convert markdown to reStructuredText), which in turn includes
the mistune library to handle the markdown parsing. Given the changes in mistune
2.0, I'm not sure how sphinxcontrib-openapi will resolve this.

Stephen