[7.0.x] Add missing cooperative constructor changelog

changelog/9488.deprecation.rst

@@ -0,0 +1,7 @@
+If custom subclasses of nodes like :class:`pytest.Item` override the
+``__init__`` method, they should take ``**kwargs``. See
+:ref:`uncooperative-constructors-deprecated` for details.
+
+Note that a deprection warning is only emitted when there is a conflict in the
+arguments pytest expected to pass. This deprecation was already part of pytest
+7.0.0rc1 but wasn't documented.