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

@@ -1,3 +1,4 @@
 pydantic>=2.10
 requests
-openai >= 1.75.0
+openai >= 1.75.0
+urllib3>=2.6.3 # not directly required, pinned by Snyk to avoid a vulnerability