Bump tornado from 6.4.1 to 6.4.2 in /docs (#587)

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: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/requirements.txt

@@ -81,7 +81,7 @@ sphinxcontrib-serializinghtml==1.1.5
 SQLAlchemy==1.4.47
 stack-data==0.6.2
 tabulate==0.9.0
-tornado==6.4.1
+tornado==6.4.2
 traitlets==5.9.0
 typing_extensions==4.5.0
 uc-micro-py==1.0.1