- 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

With this change phone numbers (any qty) can be added as `ModelForm`
property `phone_numbers_fields` - dict where "keys" are names of
phone numbers fields and "values" are attributes of fields.

* `PhoneNumberMixin` clightly reworked and moved
from `oscar.views.generic` to `oscar.forms.mixins`
* Couple tests added (based on previous tests)