bpo-33029: Fix signatures of getter and setter functions. (GH-10746)

Fix also return type for few other functions (clear, releasebuffer).
diff --git a/Objects/cellobject.c b/Objects/cellobject.c
index 6b7136c..86bebb9 100644
--- a/Objects/cellobject.c
+++ b/Objects/cellobject.c
@@ -112,7 +112,7 @@
 }
 
 static int
-cell_set_contents(PyCellObject *op, PyObject *obj)
+cell_set_contents(PyCellObject *op, PyObject *obj, void *Py_UNUSED(ignored))
 {
     Py_XINCREF(obj);
     Py_XSETREF(op->ob_ref, obj);