Bump flask-cors from 3.0.6 to 3.0.9 in /backend

Bumps [flask-cors](https://github.com/corydolphin/flask-cors) from 3.0.6 to 3.0.9.
- [Release notes](https://github.com/corydolphin/flask-cors/releases)
- [Changelog](https://github.com/corydolphin/flask-cors/blob/master/CHANGELOG.md)
- [Commits](https://github.com/corydolphin/flask-cors/compare/3.0.6...3.0.9)
Signed-off-by: Ndependabot[bot] <support@github.com>

backend/requirements.txt

@@ -2,7 +2,7 @@ six==1.11.0
 APScheduler==3.6.0
 Flask==1.0.2
 Flask-login==0.4.1
-Flask-Cors==3.0.6
+Flask-Cors==3.0.9
 pymongo==3.7.2
 bert-serving-client==1.9.1
 requests==2.20.0