Bump tornado from 6.3.3 to 6.4.1 in /requirements

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: indirect
...

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

requirements/docs.txt

@@ -60,7 +60,7 @@ sphinxcontrib-qthelp==1.0.3
     # via sphinx
 sphinxcontrib-serializinghtml==1.1.5
     # via sphinx
-tornado==6.3.3
+tornado==6.4.1
     # via livereload
 urllib3==1.26.16
     # via requests