Test get_all_ancestors against UpdateView

We previously tested against a couple of simple artificial examples,
but I didn't trust it to cover all the possible cases.

This change ensures that we're testing against a complicated real-world
example, UpdateView, which I believe to represent sufficient complexity.

tests/test_models.py

@@ -1,4 +1,8 @@
 import pytest
+from django.core.management import call_command
+from django.views.generic import UpdateView
+
+from cbv.models import Klass
 
 from .factories import InheritanceFactory, KlassFactory
 
@@ -53,3 +57,26 @@ def test_diamond(self) -> None:
         mro = d.get_all_ancestors()
 
         assert mro == [b, c, a]
+
+    def test_real(self) -> None:
+        """
+        Test the MRO of a real class hierarchy, taken from the Django's UpdateView.
+        """
+        # The version of Django that we are using for this test is 3.1
+        # because that is the version we have in our dependencies when we run tests.
+        # If this fails in future, it is probably because the version of Django
+        # has been updated and the MRO of the UpdateView has changed.
+        # In that case, feel free to update the version we're loading here.
+        call_command("loaddata", "3.1.json")
+
+        mro = Klass.objects.get(name="UpdateView").get_all_ancestors()
+
+        # For the sake of comparison, we convert the Klass objects to a tuple of strings,
+        # where the first element is the module name and the second is the class name.
+        mro_strings = [(k.module.name, k.name) for k in mro]
+
+        # The first element is the class itself, so we skip it.
+        # The last element is object, so we skip it.
+        real_ancestors = UpdateView.__mro__[1:][:-1]
+
+        assert mro_strings == [(c.__module__, c.__name__) for c in real_ancestors]