fix: example/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-WERKZEUG-6035177

Author: snyk-bot

example/requirements.txt

@@ -3,3 +3,4 @@ Flask
 gunicorn
 oic>=1.2.1
 setuptools>=65.5.1 # not directly required, pinned by Snyk to avoid a vulnerability
+werkzeug>=3.0.1 # not directly required, pinned by Snyk to avoid a vulnerability