www/py-django-filter: Update to 2.2.0

Changelog:

• Added DjangoFilterBackend.get_schema_operation_parameters() for DRF 3.10+
• OpenAPI schema generation
• Added lookup_expr to MultipleChoiceFilter
• Dropped support for EOL Python 3.4

https://github.com/carltongibson/django-filter/releases/tag/2.2.0