Bump tornado from 6.5.6 to 6.5.7 in /backend

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.5.6 to 6.5.7.
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.5.6...v6.5.7)

---
updated-dependencies:
- dependency-name: tornado
  dependency-version: 6.5.7
  dependency-type: indirect
...

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

Author: dependabot[bot]

backend/requirements.txt

@@ -235,7 +235,7 @@ textdistance==4.6.3
     # via -r requirements.in
 threadpoolctl==3.6.0
     # via scikit-learn
-tornado==6.5.6
+tornado==6.5.7
     # via
     #   django-livereload-server
     #   flower