Make fixes for deprecations in Django 3.2 and Python

- Add new "DEFAULT_AUTO_FIELD" Django 3.2 setting
- Fix deprecated (in "gettext") use of non-int numbers for selecting
  plural translations
- Provide "get_response" callable when creating middleware-factory
  instance in tests

sandbox/settings.py

     'django.contrib.staticfiles.finders.AppDirectoriesFinder',
 )
 
+# Default primary key field type
+# https://docs.djangoproject.com/en/dev/ref/settings/#default-auto-field
+DEFAULT_AUTO_FIELD = 'django.db.models.BigAutoField'
+
 # Make this unique, and don't share it with anybody.
 SECRET_KEY = env.str('SECRET_KEY', default='UajFCuyjDKmWHe29neauXzHi9eZoRXr6RMbT5JyAdPiACBP6Cra2')
 

src/oscar/apps/offer/conditions.py

             return ngettext(
                 'Buy %(delta)d more product from %(range)s',
                 'Buy %(delta)d more products from %(range)s',
-                delta
+                int(delta)
             ) % {'delta': delta, 'range': self.range}
 
     def consume_items(self, offer, basket, affected_lines):
             return ngettext(
                 'Buy %(delta)d more product from %(range)s',
                 'Buy %(delta)d more products from %(range)s',
-                delta
+                int(delta)
             ) % {'delta': delta, 'range': self.range}
 
     def is_partially_satisfied(self, offer, basket):

tests/integration/checkout/test_session.py

 from unittest import mock
 
 from django.contrib.sessions.middleware import SessionMiddleware
+from django.http import HttpResponse
 from django.test import TestCase
 from django.test.client import RequestFactory
 
     oscar.apps.checkout.utils.CheckoutSessionData
     """
 
+    @staticmethod
+    def get_response_for_test(request):
+        return HttpResponse()
+
     def setUp(self):
         request = RequestFactory().get('/')
-        SessionMiddleware().process_request(request)
+        SessionMiddleware(self.get_response_for_test).process_request(request)
         self.session_data = CheckoutSessionData(request)
 
     def test_allows_data_to_be_written_and_read_out(self):

tests/unit/checkout/test_checkout_session_data.py

 
 import pytest
 from django.contrib.sessions.middleware import SessionMiddleware
+from django.http import HttpResponse
 
 from oscar.apps.checkout.forms import ShippingAddressForm
 from oscar.apps.checkout.utils import CheckoutSessionData
 
 
+def get_response_for_test(request):
+    return HttpResponse()
+
+
 @pytest.fixture
 def csdf(rf):
     """"""
     request = rf.get('/')
-    middleware = SessionMiddleware()
+    middleware = SessionMiddleware(get_response_for_test)
     middleware.process_request(request)
     return CheckoutSessionData(request)