Update pymdown-extensions requirement from >=10.4 to >=10.21.2 (#297)

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@10.4...10.21.2)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/requirements.txt

@@ -3,4 +3,4 @@ mkdocs-material>=9.4.7
 mkdocstrings>=0.24.0
 mkdocstrings-python>=1.7.4
 markdown-exec>=1.10.3
-pymdown-extensions>=10.4
+pymdown-extensions>=10.21.2