dependabot: bump pymdown-extensions from 10.12 to 10.13 (#225)

Bumps [pymdown-extensions](https://github.com/facelessuser/pymdown-extensions) from 10.12 to 10.13.
- [Release notes](https://github.com/facelessuser/pymdown-extensions/releases)
- [Commits](facelessuser/pymdown-extensions@10.12...10.13)

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

requirements.txt

@@ -5,4 +5,4 @@ mkdocs-material==9.5.49
 mkdocs-material-extensions==1.3
 mkdocs-minify-plugin==0.8.0
 pygments==2.18.0
-pymdown-extensions==10.12
+pymdown-extensions==10.13