fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-1014645

Author: snyk-bot

requirements.txt

@@ -4,4 +4,5 @@ oauth2client==3.0.0
 requests-toolbelt==0.7.0
 python-jwt==2.0.1
 pycrypto==2.6.1
-rsa>=4.7 # not directly required, pinned by Snyk to avoid a vulnerability
+rsa>=4.7 # not directly required, pinned by Snyk to avoid a vulnerability
+urllib3>=1.25.9 # not directly required, pinned by Snyk to avoid a vulnerability