fix: appengine/flexible/websockets/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-ZIPP-7430899

Author: snyk-bot

appengine/flexible/websockets/requirements.txt

@@ -4,3 +4,4 @@ gunicorn==23.0.0
 requests==2.34.2; python_version >= '3.10'
 markupsafe==2.0.1
 Werkzeug==3.1.8; python_version >= '3.9'
+zipp>=3.19.1 # not directly required, pinned by Snyk to avoid a vulnerability