Bump mistune from 0.8.4 to 2.0.3

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>

requirements.txt

@@ -14,5 +14,5 @@ django-uuslug==1.2.0
 git+https://github.com/numbas/django-thumbs
 python-slugify==4.0.1
 six==1.15.0
-mistune==0.8.4
+mistune==2.0.3
 emoji==0.6.0