Bump tornado from 6.4.1 to 6.4.2

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.4.1 to 6.4.2.
- [Changelog](https://github.com/tornadoweb/tornado/blob/v6.4.2/docs/releases.rst)
- [Commits](https://github.com/tornadoweb/tornado/compare/v6.4.1...v6.4.2)

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

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

requirements.txt

@@ -105,7 +105,7 @@ stack-data==0.6.3
 terminado==0.18.1
 terminaltables==3.1.10
 tinycss2==1.2.1
-tornado==6.4.1
+tornado==6.4.2
 tqdm==4.66.3
 traitlets==5.14.2
 typing_extensions==4.10.0