Bump mistune from 0.8.3 to 2.0.3

Bumps [mistune](https://github.com/lepture/mistune) from 0.8.3 to 2.0.3.
- [Release notes](https://github.com/lepture/mistune/releases)
- [Changelog](https://github.com/lepture/mistune/blob/master/docs/changes.rst)
- [Commits](https://github.com/lepture/mistune/compare/v0.8.3...v2.0.3)

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

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

requirements.txt

@@ -20,7 +20,7 @@ jupyter-console==5.2.0
 jupyter-core==4.4.0
 MarkupSafe==1.0
 matplotlib==2.1.2
-mistune==0.8.3
+mistune==2.0.3
 nbconvert==5.3.1
 nbformat==4.4.0
 notebook==5.3.1