deps-dev(deps-dev): update pymdown-extensions requirement

Updates the requirements on [pymdown-extensions](https://github.com/facelessuser/pymdown-extensions) to permit the latest version.
- [Release notes](https://github.com/facelessuser/pymdown-extensions/releases)
- [Commits](facelessuser/pymdown-extensions@10.0...10.21.2)

---
updated-dependencies:
- dependency-name: pymdown-extensions
  dependency-version: 10.21.2
  dependency-type: direct:development
...

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

Author: dependabot[bot]

requirements-dev.txt

@@ -6,4 +6,4 @@ pre-commit>=3.6.0
 mkdocs>=1.6.0
 mkdocs-material>=9.5.0
 mkdocs-mermaid2-plugin>=1.1.0
-pymdown-extensions>=10.0.0
+pymdown-extensions>=10.21.2