build(deps): Bump redis from 3.5.2 to 3.5.3 in /backend (#538)

Bumps [redis](https://github.com/andymccurdy/redis-py) from 3.5.2 to 3.5.3.
- [Release notes](https://github.com/andymccurdy/redis-py/releases)
- [Changelog](https://github.com/andymccurdy/redis-py/blob/master/CHANGES)
- [Commits](redis/redis-py@3.5.2...3.5.3)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

backend/requirements.txt

@@ -9,6 +9,6 @@ google-cloud-storage==1.28.1
 gunicorn==20.0.4
 python-dateutil==2.8.1
 pytoml==0.1.21
-redis==3.5.2
+redis==3.5.3
 swagger-ui-bundle==0.0.6
 zstandard==0.13.0