Bump tornado from 6.4 to 6.4.2 in /docs

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.4 to 6.4.2.
- [Changelog](https://github.com/tornadoweb/tornado/blob/v6.4.2/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.4.0...v6.4.2)

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

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

docs/requirements.txt

@@ -33,6 +33,6 @@ sphinxcontrib-qthelp==1.0.5
 sphinxcontrib-seqdiag==3.0.0
 sphinxcontrib-serializinghtml==1.1.7
 sphinxcontrib-mermaid==0.9.2
-tornado==6.4
+tornado==6.4.2
 urllib3==2.0.4
 webcolors==1.13