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>

Author: dependabot[bot]

requirements.txt

@@ -29,7 +29,7 @@ jupyterlab-server==1.0.0
 kiwisolver==1.1.0
 MarkupSafe==1.1.1
 matplotlib==3.1.1
-mistune==0.8.4
+mistune==2.0.3
 mizani==0.5.4
 nbconvert==5.5.0
 nbformat==4.4.0