Update dependency pymdown-extensions to v10.21

| datasource | package            | from    | to    |
| ---------- | ------------------ | ------- | ----- |
| pypi       | pymdown-extensions | 10.20.1 | 10.21 |


Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

docs/requirements.txt

@@ -5,7 +5,7 @@ mkdocs-material==9.7.1
 
 # Markdown extensions
 Pygments==2.19.2
-pymdown-extensions==10.20.1
+pymdown-extensions==10.21
 
 # MkDocs plugins
 mkdocs-material-extensions==1.3.1