check mistune pin

[dbkeitel]

We were on "<2.0.0" since #360 and now #435 changes it to "==0.8.4" which happents to be equivalent as per
https://pypi.org/project/mistune/#history

But they're now at 2.0.4 stable and 3.0.0 alpha, so we should check what's going on with cross-dependencies.

[Rodrigo-Tenorio]

For the record: I unpinned everything in docs and let pip decide on the proper combination.
I may have missed that one as well, but that's what I intended to do.

[dbkeitel]

This is still the version one ends up when unpinning and letting sphinx & friends do their thing. It actually comes in via m2r2 and removing that pin is a long-standing work-in-progress of theirs tracked here: CrossNox/m2r2#47

Nothing for us to do.