gh-113318: Support @deleter as a separate function in Argument Clinic

[serhiy-storchaka]

Used alone, the @deleter directive defines a function which deletes the attribute. It can be combined with a @setter defined in a separate block: the setter slot of the PyGetSetDef entry is then filled with a generated dispatcher which calls the setter or the deleter depending on whether the value is NULL.

/*[clinic input]
@setter
Test.settable_deletable
    value: int
[clinic start generated code]*/

/*[clinic input]
@deleter
Test.settable_deletable
[clinic start generated code]*/

The two impl functions get distinct C names (..._set_impl and ..._del_impl), so neither has to handle the other's case, and a setter that has a separate deleter no longer contains a dead if (arg == NULL) check for a case that the dispatcher never passes to it.

function.__annotations__ is converted as the first user: the deleter is a separate @deleter block, and the setter is left with only the None-or-dict logic.

There are now 36 @setter and 6 @deleter uses in the tree. 30 setters reject deletion with the generated AttributeError, 5 are combined with @deleter in the same block and handle NULL themselves (frame.f_trace, _ctypes.CType_Type.__pointer_type__, _ctypes.CFuncPtr.errcheck, .restype, .argtypes), and function.__annotations__ uses the new separate @deleter.

• Issue: Argument Clinic: Better internal implementation for @getter and @setter #113318