fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PYMDOWNEXTENSIONS-12427119

Author: snyk-bot

requirements.txt

@@ -24,7 +24,7 @@ markdown>=3.2
 mkdocs>=1.4.2
 mkdocs-material-extensions>=1.1
 pygments>=2.14
-pymdown-extensions>=9.9.1
+pymdown-extensions>=10.16.1
 
 # Requirements for plugins
 colorama>=0.4