Don't use accidental pytest API, fix tests for pytest 6.2+

See pytest-dev/pytest#8282

tests/test_ordering.py

@@ -12,9 +12,9 @@ def item_names_for(testdir):
     def _item_names_for(tests_content):
         # some strange code to extract sorted items
         items = testdir.getitems(tests_content)
-        hook = testdir.config.hook
+        hook = items[0].config.hook
         hook.pytest_collection_modifyitems(session=items[0].session,
-                                           config=testdir.config, items=items)
+                                           config=items[0].config, items=items)
         return [item.name for item in items]
 
     return _item_names_for