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

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] <[email protected]>

docs/.sphinx/pinned-requirements.txt

@@ -30,5 +30,5 @@ sphinxcontrib-htmlhelp==2.0.5
 sphinxcontrib-jsmath==1.0.1
 sphinxcontrib-qthelp==1.0.8
 sphinxcontrib-serializinghtml==1.1.10
-tornado==6.4.1
+tornado==6.4.2
 urllib3==1.26.20