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

@@ -59,7 +59,7 @@ markdown==3.3.4; python_version >= "3.6"
 markupsafe==2.0.1; python_version >= "3.6"
 matplotlib-inline==0.1.3; python_version >= "3.7"
 matplotlib==3.4.3; python_version >= "3.7"
-mistune==0.8.4; python_version >= "3.7"
+mistune==2.0.3; python_version >= "3.7"
 nbclient==0.5.4; python_full_version >= "3.6.1" and python_version >= "3.7"
 nbconvert==6.2.0; python_version >= "3.7"
 nbformat==5.1.3; python_full_version >= "3.6.1" and python_version >= "3.7"