Bump tornado from 6.3.3 to 6.4.1 in /examples/server/tornado

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](https://github.com/tornadoweb/tornado/compare/v6.3.3...v6.4.1)

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

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

examples/server/tornado/requirements.txt

@@ -1,4 +1,4 @@
-tornado==6.3.3
+tornado==6.4.1
 python-engineio
 python_socketio
 six==1.10.0