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/rtd-requirements.txt

@@ -33,7 +33,7 @@ sphinxcontrib-htmlhelp==2.0.0
 sphinxcontrib-jsmath==1.0.1
 sphinxcontrib-qthelp==1.0.3
 sphinxcontrib-serializinghtml==1.1.5
-tornado==6.3.3
+tornado==6.4.1
 traitlets==5.1.0
 urllib3==1.26.18
 wcwidth==0.2.5