Bump tornado from 5.1.1 to 6.4.1

Bumps [tornado](https://github.com/tornadoweb/tornado) from 5.1.1 to 6.4.1.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v5.1.1...v6.4.1)

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

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

requirements.txt

@@ -10,4 +10,4 @@ pymdown-extensions==6.0
 PyYAML==5.3.1
 six==1.11.0
 smmap2==2.0.5
-tornado==5.1.1
+tornado==6.4.1