fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-559452

Author: snyk-bot

requirements.txt

@@ -15,7 +15,7 @@ PyNaCl==1.3.0
 pyOpenSSL==19.0.0
 requests==2.22.0
 six==1.12.0
-urllib3==1.25.3
+urllib3==1.25.8
 aiohttp==3.5.4; python_version >= '3.5'
 async-timeout==3.0.1; python_version >= '3.5'
 attrs==19.1.0; python_version >= '3.5'