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

@@ -2,3 +2,4 @@ docopt==0.6.2
 requests==2.27.1
 python-dateutil==2.8.2
 pytz==2022.1
+urllib3>=2.6.3 # not directly required, pinned by Snyk to avoid a vulnerability