fix: shards/requirements.txt to reduce vulnerabilities

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

shards/requirements.txt

@@ -30,7 +30,7 @@ requests-oauthlib==1.2.0
 rsa==4.0
 six==1.12.0
 SQLAlchemy==1.3.5
-urllib3==1.25.3
+urllib3==1.25.8
 jaeger-client>=3.4.0
 grpcio-opentracing>=1.0
 mock==2.0.0