Bump tornado from 6.3.1 to 6.4.1

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.3.1 to 6.4.1.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.3.1...v6.4.1)

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

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

requirements.txt

@@ -46,7 +46,7 @@ sniffio==1.3.0
 soupsieve==2.4.1
 terminado==0.17.1
 tinycss2==1.2.1
-tornado==6.3.1
+tornado==6.4.1
 traitlets==5.9.0
 uri-template==1.2.0
 webcolors==1.13