Bump tornado from 6.1 to 6.4.2

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.1 to 6.4.2.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.1.0...v6.4.2)

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

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

Author: dependabot[bot]

Pipfile

@@ -73,7 +73,7 @@ sphinxcontrib-jsmath = "==1.0.1"
 sphinxcontrib-qthelp = "==1.0.3"
 sphinxcontrib-serializinghtml = "==1.1.4"
 toml = "==0.10.2"
-tornado = "==6.1"
+tornado = "==6.4.2"
 tqdm = "==4.57.0"
 typed-ast = "==1.5.4"
 urllib3 = "==1.26.5"