chore(deps): bump tornado from 6.1 to 6.4

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.1 to 6.4.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.1.0...v6.4.0)

---
updated-dependencies:
- dependency-name: tornado
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

requirements.txt

@@ -88,7 +88,7 @@ stevedore==5.2.0; python_version >= '3.8'
 tabulate==0.9.0; python_version >= '3.7'
 toml==0.10.2; python_version >= '2.6' and python_version not in '3.0, 3.1, 3.2, 3.3'
 tomli==2.0.1; python_version >= '3.7'
-tornado==6.1; python_version >= '3.5'
+tornado==6.4; python_version >= '3.5'
 traitlets==5.14.3; python_version >= '3.7'
 trio==0.19.0; python_version >= '3.7'
 trio-websocket==0.9.2; python_version >= '3.7'