Bump tornado from 6.3.3 to 6.4.1 in /docs

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.3.3 to 6.4.1.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.3.3...v6.4.1)

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

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

docs/requirements.txt

@@ -26,7 +26,7 @@ regex==2021.8.3
 requests==2.32.0
 six==1.16.0
 toml==0.10.2
-tornado==6.3.3
+tornado==6.4.1
 tqdm==4.66.3
 ujson==5.4.0
 urllib3==1.26.19