Bump tornado from 6.3.3 to 6.4.1 in /docs (#660)

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>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/requirements.txt

@@ -36,5 +36,5 @@ sphinx-notfound-page==0.8.3
 sphinx-prompt==1.6.0
 sphinx-rtd-theme==1.2.2
 sphinx-tabs==3.4.1
-tornado==6.3.3
+tornado==6.4.1
 urllib3==2.0.7