fix: test-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

test-requirements.txt

@@ -8,3 +8,4 @@ mock
 sphinx
 sphinx_rtd_theme
 setuptools>=78.1.1 # not directly required, pinned by Snyk to avoid a vulnerability
+pygments>=2.20.0 # not directly required, pinned by Snyk to avoid a vulnerability