Bump mistune from 0.7.1 to 0.8.1

Bumps [mistune](https://github.com/lepture/mistune) from 0.7.1 to 0.8.1.
- [Release notes](https://github.com/lepture/mistune/releases)
- [Changelog](https://github.com/lepture/mistune/blob/v0.8.1/CHANGES.rst)
- [Commits](https://github.com/lepture/mistune/compare/v0.7.1...v0.8.1)

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

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

requirements.txt

@@ -17,7 +17,7 @@ jupyter-console==4.0.1
 jupyter-core==4.0.4
 MarkupSafe==0.23
 matplotlib==1.4.3
-mistune==0.7.1
+mistune==0.8.1
 mock==1.3.0
 nbconvert==4.0.0
 nbformat==4.0.0