Bump tornado from 6.4 to 6.4.1 in /images/hub

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.4 to 6.4.1.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.4.0...v6.4.1)

---
updated-dependencies:
- dependency-name: tornado
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

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