Bump mistune from 0.8.4 to 2.0.3 in /doc

Bumps [mistune](https://github.com/lepture/mistune) from 0.8.4 to 2.0.3.
- [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...v2.0.3)

---
updated-dependencies:
- dependency-name: mistune
  dependency-type: direct:production
...

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

doc/requirements.txt

@@ -4,7 +4,7 @@ ipykernel==6.7.0
 jinja2==3.0.3
 sphinx==2.2.2
 m2r2==0.3.1
-mistune==0.8.4
+mistune==2.0.3
 numba==0.53.1
 networkx==2.6.3
 numpy==1.19.2