Bump flask-cors from 3.0.3 to 3.0.9 in /backend/flask

Bumps [flask-cors](https://github.com/corydolphin/flask-cors) from 3.0.3 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](corydolphin/flask-cors@3.0.3...3.0.9)

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

Author: dependabot[bot]

backend/flask/requirements.txt

@@ -3,7 +3,7 @@ Flask-Migrate==2.1.0
 Flask-SQLAlchemy==2.2
 Flask-Script==2.0.5
 Flask-RESTful==0.3.6
-flask-cors==3.0.3
+flask-cors==3.0.9
 psycopg2==2.7.3
 PyJWT==1.5.2
 requests==2.18.4