build(deps): bump tornado from 6.4.1 to 6.4.2 in /docs

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: indirect
...

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

docs/requirements.txt

@@ -113,7 +113,7 @@ sphinxcontrib-spelling==7.7.0
     # via -r requirements.in
 stevedore==3.5.2
     # via doc8
-tornado==6.4.1
+tornado==6.4.2
     # via livereload
 treelib==1.6.4
     # via -r ../tools/lib-alert-tree/requirements.txt