build(deps): bump tornado from 6.2 to 6.4.1

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.2 to 6.4.1.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.2.0...v6.4.1)

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

Signed-off-by: dependabot[bot] <[email protected]>

requirements-docs.txt

@@ -62,7 +62,7 @@ sphinxcontrib-qthelp==1.0.3
 sphinxcontrib-serializinghtml==1.1.5
 stack-data==0.3.0
 tinycss2==1.1.1
-tornado==6.2
+tornado==6.4.1
 tqdm==4.64.0
 traitlets==5.3.0
 urllib3==1.26.19