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] <support@github.com>

Author: dependabot[bot]

docs/requirements.txt

@@ -68,7 +68,7 @@ six==1.16.0
 terminado==0.10.1
 testpath==0.5.0
 toml==0.10.2
-tornado==6.3.3
+tornado==6.4.2
 traitlets==5.0.5
 wcwidth==0.2.5
 webencodings==0.5.1