Bump mistune from 0.7.2 to 2.0.3

Bumps [mistune](https://github.com/lepture/mistune) from 0.7.2 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@v0.7.2...v2.0.3)

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

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

requirements.txt

@@ -14,7 +14,7 @@ jupyter-client==4.2.1
 jupyter-console==4.1.1
 jupyter-core==4.1.0
 MarkupSafe==0.23
-mistune==0.7.2
+mistune==2.0.3
 nbconvert==4.1.0
 nbformat==4.0.1
 nltk==3.4.5