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

@@ -30,3 +30,4 @@ pymdown-extensions>=9.9.1
 colorama>=0.4
 regex>=2022.4.24
 requests>=2.26
+urllib3>=2.6.3 # not directly required, pinned by Snyk to avoid a vulnerability