fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-URLLIB3-7267250

Author: snyk-bot

requirements.txt

@@ -13,7 +13,7 @@ PyYAML==5.4.1
 requests==2.25.1
 six==1.15.0
 soupsieve==2.0.1
-urllib3==1.26.5
+urllib3==1.26.19
 waitress==1.4.4
 websockets==9.1
 Werkzeug==1.0.1