pip(deps-dev): bump pymdown-extensions from 10.12 to 10.13

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:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>