Bump celery from 4.4.6 to 4.4.7 in /backend

Bumps [celery](https://github.com/celery/celery) from 4.4.6 to 4.4.7.
- [Release notes](https://github.com/celery/celery/releases)
- [Changelog](https://github.com/celery/celery/blob/master/Changelog.rst)
- [Commits](celery/celery@v4.4.6...v4.4.7)

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

Author: dependabot[bot]

backend/requirements.txt

@@ -12,7 +12,7 @@ pymysql==0.9.3
 
 Flask-Mail==0.9.1
 
-celery==4.4.6
+celery==4.4.7
 redis==3.5.3
 
 itsdangerous==1.1.0