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](tornadoweb/tornado@v6.4.1...v6.4.2)

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

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

requirements.txt

@@ -159,7 +159,7 @@ sphinxcontrib-serializinghtml==1.1.9
     # via sphinx
 tinycss2==1.2.1
     # via nbconvert
-tornado==6.4.1
+tornado==6.4.2
     # via jupyter-client
 traitlets==5.14.0
     # via