Support Django 4.x

Django 4.x is now in unstable, and there is an error when building the freedombox package:

____________ ERROR at setup of TestSetupMiddleware.test_module_view ____________

load_cfg = <module 'plinth.cfg' from '/build/freedombox-22.13/plinth/cfg.py'>

    @staticmethod
    @pytest.fixture(name='middleware')
    def fixture_middleware(load_cfg):
        """Fixture for returning middleware."""
>       return SetupMiddleware()
E       TypeError: MiddlewareMixin.__init__() missing 1 required positional argument: 'get_response'

plinth/tests/test_middleware.py:58: TypeError

From https://docs.djangoproject.com/en/4.0/releases/4.0/:

The get_response argument for django.utils.deprecation.MiddlewareMixin.__init__() is required and doesn’t accept None.