Bump mistune from 0.7.3 to 0.8.1

Bumps [mistune](https://github.com/lepture/mistune) from 0.7.3 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@v0.7.3...v0.8.1)

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

requirements.txt

@@ -103,7 +103,7 @@ MarkupSafe==0.23
 matplotlib==2.0.2
 mccabe==0.6.1
 mechanize==0.3.3
-mistune==0.7.3
+mistune==0.8.1
 mock==2.0.0
 modulegraph==0.10.4
 mpmath==0.19