fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-CERTIFI-3164749
- https://snyk.io/vuln/SNYK-PYTHON-CERTIFI-5805047
- https://snyk.io/vuln/SNYK-PYTHON-REQUESTS-5595532

requirements.txt

@@ -16,3 +16,5 @@ tiktoken==0.5.2
 tornado>=6.3.3 # not directly required, pinned by Snyk to avoid a vulnerability
 validators>=0.21.0 # not directly required, pinned by Snyk to avoid a vulnerability
 wikipedia==1.4.0
+certifi>=2023.7.22 # not directly required, pinned by Snyk to avoid a vulnerability
+requests>=2.31.0 # not directly required, pinned by Snyk to avoid a vulnerability