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: direct:development
...

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

Author: dependabot[bot]

requirements_dev.txt

@@ -310,7 +310,7 @@ stack-data==0.6.3
     # via ipython
 toml==0.10.2
     # via pyee (pyproject.toml)
-tornado==6.4.1
+tornado==6.4.2
     # via
     #   ipykernel
     #   jupyter-client