Merge pull request #3429 from jupyterhub/dependabot/pip/images/hub/tornado-6.4.1

Bump tornado from 6.4 to 6.4.1 in /images/hub

Author: manics

images/hub/requirements.txt

@@ -216,7 +216,7 @@ statsd==4.0.1
     # via -r requirements.in
 text-unidecode==1.3
     # via python-slugify
-tornado==6.4
+tornado==6.4.1
     # via
     #   jupyterhub
     #   jupyterhub-idle-culler