Merge pull request #3582 from jupyterhub/dependabot/pip/images/hub/tornado-6.4.2

Bump tornado from 6.4.1 to 6.4.2 in /images/hub

Author: manics

images/hub/requirements.txt

@@ -229,7 +229,7 @@ statsd==4.0.1
     # via -r unfrozen/requirements.txt
 text-unidecode==1.3
     # via python-slugify
-tornado==6.4.1
+tornado==6.4.2
     # via
     #   jupyterhub
     #   jupyterhub-idle-culler