Bump django from 2.1.7 to 2.1.9

Bumps [django](https://github.com/django/django) from 2.1.7 to 2.1.9.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/2.1.7...2.1.9)

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

requirements.txt

@@ -5,7 +5,7 @@ APScheduler==3.2.0
 billiard==3.5.0.3
 celery==4.1.0
 coreapi==2.0.9
-Django==2.1.7
+Django==2.1.9
 django-appconf==1.0.2
 django-codemirror2==0.2
 django-colorfield==0.1.15