Bump django from 2.2.4 to 2.2.20 in /requirements

Bumps [django](https://github.com/django/django) from 2.2.4 to 2.2.20.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.4...2.2.20)

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

requirements/base.txt

@@ -4,7 +4,7 @@ cryptocompare==0.6
 cryptography==2.3
 celery==4.4.0
 django-celery-beat==1.1.1
-django==2.2.4
+django==2.2.20
 django-csp==3.7
 django-cors-headers==2.4.0
 django-filter==2.0.0