fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-14896210

Author: snyk-bot

requirements.txt

@@ -6,3 +6,4 @@ tenacity >= 8.0.1
 openai >= 1.74.1
 sseclient-py >= 1.8.0
 validators == 0.20.0
+urllib3>=2.6.3 # not directly required, pinned by Snyk to avoid a vulnerability