Bump mistune from 0.8.4 to 2.0.0

Bumps [mistune](https://github.com/lepture/mistune) from 0.8.4 to 2.0.0.
- [Release notes](https://github.com/lepture/mistune/releases)
- [Changelog](https://github.com/lepture/mistune/blob/master/docs/changes.rst)
- [Commits](lepture/mistune@v0.8.4...v2.0.0)

---
updated-dependencies:
- dependency-name: mistune
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

docs/requirements.txt

@@ -1,4 +1,4 @@
 sphinx==4.3.2
 sphinx_rtd_theme==1.0.0
 m2r2==0.3.2
-mistune==0.8.4
+mistune==2.0.0