python/cpython#104389: Add 'unused' keyword to Argument Clinic C conv…

…erters (python/cpython#104390)

Use the unused keyword param in the converter to explicitly
mark an argument as unused:

    /*[clinic input]
    SomeBaseClass.stubmethod
        flag: bool(unused=True)
    [clinic start generated code]*/

Doc/howto/clinic.rst

@@ -775,6 +775,9 @@ All Argument Clinic converters accept the following arguments:
     because :pep:`8` mandates that the Python library may not use
     annotations.
 
+  ``unused``
+    Wrap the argument with :c:macro:`Py_UNUSED` in the impl function signature.
+
 In addition, some converters accept additional arguments.  Here is a list
 of these arguments, along with their meanings: