Bump django from 2.2.13 to 2.2.20 in /backend

Bumps [django](https://github.com/django/django) from 2.2.13 to 2.2.20.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.13...2.2.20)

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

Author: dependabot[bot]

backend/requirements.txt

@@ -1,7 +1,7 @@
 coreapi==2.3.3
 coreschema==0.0.4
 coverage==4.5.4
-Django==2.2.13
+Django==2.2.20
 django-cors-headers==3.0.2
 django-debug-toolbar==2.0
 django-nose==1.4.6