Fixes#7417.
Fixes all these issues seen with `tox -e py38-django31`:
```
/Users/chainz/Documents/Projects/django-rest-framework/tests/test_request.py:208: RemovedInDjango40Warning: Passing None for the middleware get_response argument is deprecated.
SessionMiddleware().process_request(self.wrapped_request)
tests/test_requests_client.py: 1 test with warning
tests/test_testing.py: 4 tests with warnings
tests/test_throttling.py: 1 test with warning
tests/authentication/test_authentication.py: 4 tests with warnings
tests/browsable_api/test_browsable_api.py: 4 tests with warnings
/Users/chainz/Documents/Projects/django-rest-framework/rest_framework/authentication.py:139: RemovedInDjango40Warning: Passing None for the middleware get_response argument is deprecated.
check = CSRFCheck()
```
* url() is deprecated in Django 3.1
* update given feedbacks on url() is deprecated in Django 3.1
* Fix test_urlpatterns.py to continue testing mixed re_path() and path()
* Fix one missed reference
Co-authored-by: sanjusci <sanju.sci9@gmail.com>
Django 3.1 adds a new generic JSONField to replace the PostgreSQL-specific one. This adds support for the new field type, which should behave the same as the existing PostgreSQL field.
Django's new JSONField also includes support for a custom "decoder", so add support for that in the serializer field.
* Make `NullBooleanField` subclass `BooleanField`
This removes a lot of the redundancy that was in place becuase we
were not doing this. This maintains the `None` initial value that
was previously present, as well as disallowing `allow_null` to be
passed in.
* Remove special case for mapping `NullBooleanField`
In newer versions of Django, the `NullBooleanField` is handled the
same way as a `BooleanField(null=True)`. Given that we also support
that combination, and that our own `NullBooleanField` behaves in the
same manner, it makes sense to remove the special casing that exists
for it.
* Add test for BooleanField(null=True, choices)
* Remove special case for NullBooleanField
* Adjust mapping tests for NullBooleanField
* Fixed linting error
* Raise deprecation warning when NullBooleanField is used
* Fix linting issue in imports
* Fix ModelSerializer unique_together field sources
Updates ModelSerializer to check for serializer fields that map to the
model field sources in the unique_together lists.
* Ensure field name ordering consistency
* SearchFilter to support Custom query Transforms
Since Some fields support `__` as a custom Transform for query lookups we needed to update the m2m checking code to handle search_fields that contain __ that are not relationships.
* Update documentation on SearchFilter to include references to JSON and HStore Fields.
