build(deps): bump mistletoe from 1.2.1 to 1.3.0

Bumps [mistletoe](https://github.com/miyuchina/mistletoe) from 1.2.1 to 1.3.0.
- [Release notes](https://github.com/miyuchina/mistletoe/releases)
- [Changelog](https://github.com/miyuchina/mistletoe/blob/master/cutting-a-release.md)
- [Commits](miyuchina/mistletoe@v1.2.1...v1.3.0)

---
updated-dependencies:
- dependency-name: mistletoe
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

requirements/optional.txt

@@ -13,7 +13,7 @@ fluent.syntax==0.19.0 # Fluent
 # Format support
 iniparse==0.5        # INI
 # Format support
-mistletoe==1.2.1       # Markdown
+mistletoe==1.3.0       # Markdown
 # Format support
 phply==1.2.6         # PHP
 pyenchant==3.2.2     # spellcheck