chore(deps): bump tornado from 6.5.5 to 6.5.7

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.5.5 to 6.5.7.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.5.5...v6.5.7)

---
updated-dependencies:
- dependency-name: tornado
  dependency-version: 6.5.7
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/requirements.txt

@@ -552,7 +552,7 @@ tinycss2==1.4.0
     # via bleach
 tomlkit==0.14.0
     # via pylint
-tornado==6.5.5
+tornado==6.5.7
     # via
     #   ipykernel
     #   jupyter-client