[issue2551198] mistune 2.0 support

New submission from John Rouillard:

Only mistume 1.x is supported. Mistune 2.0 support wasn't straight forward.
I opened a ticket with mistune and they just responded.

https://github.com/lepture/mistune/issues/290#issuecomment-1086333175

so incorporate their changes into the code base.

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components: Web interface
messages: 7465
nosy: rouilj
severity: normal
status: new
title: mistune 2.0 support
type: rfe

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