fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-VALIDATORS-6008990

requirements.txt

@@ -10,3 +10,4 @@ streamlit==1.27.2
 streamlit-feedback==0.1.2
 tiktoken==0.5.1
 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