fix: shards/requirements.txt to reduce vulnerabilities (#2536)

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-RSA-570831

shards/requirements.txt

@@ -27,7 +27,7 @@ python-dotenv==0.10.3
 pytz==2019.1
 requests==2.22.0
 requests-oauthlib==1.2.0
-rsa==4.0
+rsa==4.1
 six==1.12.0
 SQLAlchemy==1.3.5
 ujson==2.0.1