build(deps): Bump redis from 3.2.1 to 3.3.5 in /backend

Bumps [redis](https://github.com/andymccurdy/redis-py) from 3.2.1 to 3.3.5.
- [Release notes](https://github.com/andymccurdy/redis-py/releases)
- [Changelog](https://github.com/andymccurdy/redis-py/blob/master/CHANGES)
- [Commits](redis/redis-py@3.2.1...3.3.5)

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

Author: dependabot-preview[bot]

backend/requirements.txt

@@ -9,6 +9,6 @@ gunicorn==19.9.0
 python-dateutil==2.8.0
 pytoml==0.1.21
 fakeredis==1.0.3
-redis==3.2.1
+redis==3.3.5
 swagger-ui-bundle==0.0.5
 zstandard==0.11.1