Bump tornado from 6.3.3 to 6.4.2

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.3.3 to 6.4.2.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](https://github.com/tornadoweb/tornado/compare/v6.3.3...v6.4.2)

---
updated-dependencies:
- dependency-name: tornado
  dependency-version: 6.4.2
  dependency-type: direct:production
...

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

requirements.txt

@@ -107,7 +107,7 @@ stack-data==0.2.0
 terminado==0.17.1
 terminaltables==3.1.10
 tinycss2==1.2.1
-tornado==6.3.3
+tornado==6.4.2
 tqdm==4.67.1
 traitlets==5.7.1
 typing_extensions==4.9.0