Bump tornado from 6.3.3 to 6.4.2 in /docs

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

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

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

docs/notebook_requirements.txt

@@ -48,7 +48,7 @@ soupsieve==2.3.2.post1
 terminado==0.8.3
 testpath==0.3.1
 tinycss2==1.1.1
-tornado==6.3.3
+tornado==6.4.2
 traitlets==5.0.0
 wcwidth==0.1.7
 webencodings==0.5.1