build(deps): bump tornado from 5.1.1 to 6.4.1

Bumps [tornado](https://github.com/tornadoweb/tornado) from 5.1.1 to 6.4.1.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v5.1.1...v6.4.1)

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

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

requirements-production.txt

@@ -530,7 +530,7 @@ testpath==0.5.0
     # via nbconvert
 text-unidecode==1.3
     # via python-slugify
-tornado==5.1.1
+tornado==6.4.1
     # via
     #   invenio-previewer
     #   jupyter-client