build(deps): bump tornado from 6.3.3 to 6.4

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.3.3 to 6.4.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.3.3...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]>

Author: dependabot[bot]

requirements.txt

@@ -19,4 +19,4 @@ Pygments==2.17.2
 pymdown-extensions==10.5
 PyYAML==6.0.1
 six==1.16.0
-tornado==6.3.3
+tornado==6.4