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](https://github.com/carltongibson/django-filter/compare/2.1.0...2.2.0

)

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