Bump tornado from 6.4.1 to 6.4.2 in /images/hub

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.4.1 to 6.4.2.
- [Changelog](https://github.com/tornadoweb/tornado/blob/v6.4.2/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.4.1...v6.4.2)

---
updated-dependencies:
- dependency-name: tornado
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

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