Bump mistune from 0.8.4 to 2.0.3 (#14)

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
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -32,7 +32,7 @@ jupyter-client==7.3.4
 jupyter-core==4.10.0
 jupyterlab-pygments==0.2.2
 MarkupSafe==2.1.1
-mistune==0.8.4
+mistune==2.0.3
 nbclient==0.6.6
 nbconvert==6.5.0
 nbformat==5.4.0