fix: python/docs/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PYGMENTS-15746419

Author: snyk-bot

python/docs/requirements.txt

@@ -3,3 +3,4 @@ jinja2==3.0.0
 sphinx==2.3.1
 sphinx_rtd_theme==0.4.3
 sphinxcontrib-napoleon==0.7
+pygments>=2.20.0 # not directly required, pinned by Snyk to avoid a vulnerability