Fix classmethod check for mixin case

Fixes regression in #598.

It looks like this was actually a latent failure in the way
`_classmethod_is_defined_at_leaf` was picking the 'super method' - it
would iterate further up the tree despite coming across the next method
in the resolution chain. Also by not using `__mro__` the order of the
base classes wasn't strictly being observed, although I don't think that
affects my mixin case.

Added a test that fails before and passes after.

Closes pytest-dev/pytest-django#618.

docs/changelog.rst

@@ -4,6 +4,12 @@ Changelog
 unreleased
 ------------------
 
+Bug fixes
+^^^^^^^^^
+
+* Fixed test for classmethod with Django TestCases again (#618,
+  introduced in #598 (3.3.0)).
+
 Compatibility
 ^^^^^^^^^^^^^
 

pytest_django/plugin.py

@@ -263,9 +263,10 @@ def pytest_configure():
 def _classmethod_is_defined_at_leaf(cls, method_name):
     super_method = None
 
-    for base_cls in cls.__bases__:
+    for base_cls in cls.__mro__[1:]:  # pragma: no branch
         if hasattr(base_cls, method_name):
             super_method = getattr(base_cls, method_name)
+            break
 
     assert super_method is not None, (
         '%s could not be found in base class' % method_name)

tests/test_unittest.py

@@ -176,6 +176,34 @@ def test_bar21(self):
         ])
         assert result.ret == 0
 
+    def test_setUpClass_mixin(self, django_testdir):
+        django_testdir.create_test_module('''
+            from django.test import TestCase
+
+            class TheMixin(object):
+                @classmethod
+                def setUpClass(cls):
+                    super(TheMixin, cls).setUpClass()
+
+
+            class TestFoo(TheMixin, TestCase):
+                def test_foo(self):
+                    pass
+
+
+            class TestBar(TheMixin, TestCase):
+                def test_bar(self):
+                    pass
+        ''')
+
+        result = django_testdir.runpytest_subprocess('-v', '-s', '--pdb')
+        result.stdout.fnmatch_lines([
+            "*TestFoo::test_foo Creating test database for*",
+            "PASSED",
+            "*TestBar::test_bar PASSED*",
+        ])
+        assert result.ret == 0
+
     def test_setUpClass_skip(self, django_testdir):
         django_testdir.create_test_module('''
             from django.test import TestCase