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](lepture/mistune@v0.7.1...v0.8.1)

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

requirements.txt

@@ -18,7 +18,7 @@ MarkupSafe==0.23
 marshmallow==2.2.0
 mimerender==0.5.5
 misaka==1.0.2
-mistune==0.7.1
+mistune==0.8.1
 pycparser==2.14
 Pygments==2.0.2
 python-dateutil==2.4.2