Bump django-filter from 2.1.0 to 2.2.0

Bumps [django-filter](https://github.com/carltongibson/django-filter) from 2.1.0 to 2.2.0.
- [Release notes](https://github.com/carltongibson/django-filter/releases)
- [Changelog](https://github.com/carltongibson/django-filter/blob/master/CHANGES.rst)
- [Commits](carltongibson/django-filter@2.1.0...2.2.0)

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

requirements/base.txt

@@ -5,7 +5,7 @@ dj-static==0.0.6
 django-csp==3.5
 django-csp-reports==1.3
 django-crispy-forms==1.7.2
-django-filter==2.1.0
+django-filter==2.2.0
 djangorestframework==3.10.0
 django-secure==1.0.1
 django-toolbelt==0.0.1