fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-10390193
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-10390194

Author: snyk-bot

requirements.txt

@@ -5,3 +5,4 @@ django-phonenumber-field==6.1.0
 twilio>=7
 wheel>=0.22.0
 setuptools>=36.2
+urllib3>=2.5.0 # not directly required, pinned by Snyk to avoid a vulnerability