docs/requirements.txt: Specify mistune version

[SamB]

mistune 2.x seems too new for us?

Summary

The readthedocs build broke; see https://readthedocs.org/projects/d20/builds/18687945/ for details.

I'm hoping this might fix it, but I haven't actually tried it.

Checklist

PR Type

• This PR is a code change that implements a feature request.
• This PR fixes an issue.
• This PR adds a new feature that is not an open feature request.
• This PR is not a code change (e.g. documentation, README, ...)

Other

• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• If code changes were made then they have been tested.
• I have updated the documentation to reflect the changes.

[zhudotexe]

Thanks for the PR! Rather than pinning mistune, maybe we can upgrade to Sphinx v4? You can test it locally with cd docs; pip install -r requirements.txt; make html.