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](https://github.com/lepture/mistune/compare/v0.8.4...v2.0.3

)

---
updated-dependencies:
- dependency-name: mistune
  dependency-type: indirect
...

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