Re-enable django-dbbackup

...and bump to 3.3.0, which does support Django 3.0 [1].

[1] Archmonger/django-dbbackup#314 (comment)

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

requirements-dev.txt

@@ -2,5 +2,5 @@ Django~=3.1.0
 djangorestframework~=3.12.0
 django-filter~=2.4.0
 django-debug-toolbar~=3.2.0
-# django-dbbackup~=3.2.0
+django-dbbackup~=3.3.0
 -r requirements-test.txt