Bump tornado

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.0.3 to 6.4.1.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.0.3...v6.4.1)

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

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

docs/samples/v1alpha2/custom/kfserving-custom-model/model-server/requirements.txt

@@ -36,6 +36,6 @@ six==1.14.0
 table-logger==0.3.6
 torch==1.4.0
 torchvision==0.5.0
-tornado==6.0.3
+tornado==6.4.1
 urllib3==1.25.8
 websocket-client==0.57.0