build(deps): bump pymdown-extensions from 10.20.1 to 10.21.3

Bumps [pymdown-extensions](https://github.com/facelessuser/pymdown-extensions) from 10.20.1 to 10.21.3.
- [Release notes](https://github.com/facelessuser/pymdown-extensions/releases)
- [Commits](https://github.com/facelessuser/pymdown-extensions/commits/10.21.3)

---
updated-dependencies:
- dependency-name: pymdown-extensions
  dependency-version: 10.21.3
  dependency-type: indirect
...

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Author: dependabot[bot]