Fix regression introduced in 96f5fb1.

The previous fix did not properly check the full MRO chain when looking
for the super method, which then failed when used with mixins that does
not define setUpClass/tearDownClass.

This commit fixes issue pytest-dev#280.

pytest_django/plugin.py

@@ -251,7 +251,13 @@ def pytest_configure():
 
 
 def _method_is_defined_at_leaf(cls, method_name):
-    return getattr(cls.__base__, method_name).__func__ is not getattr(cls, method_name).__func__
+    super_method = None
+
+    for base_cls in cls.__bases__:
+        if hasattr(base_cls, method_name):
+            super_method = getattr(base_cls, method_name)
+
+    return getattr(cls, method_name).__func__ is not super_method.__func__
 
 
 _disabled_classmethods = {}

tests/test_unittest.py

@@ -124,7 +124,13 @@ def test_multi_inheritance_setUpClass(self, django_testdir):
             from django.test import TestCase
             from .app.models import Item
 
-            class TestA(TestCase):
+            # Using a mixin is a regression test, see #280 for more details:
+            # https://github.com/pytest-dev/pytest-django/issues/280
+
+            class SomeMixin(object):
+                pass
+
+            class TestA(SomeMixin, TestCase):
                 expected_state = ['A']
                 state = []