Bump tornado from 6.2 to 6.4.2

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.2 to 6.4.2.
- [Changelog](https://github.com/tornadoweb/tornado/blob/v6.4.2/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.2.0...v6.4.2)

---
updated-dependencies:
- dependency-name: tornado
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>