gh-106572: Convert PyObject_DelAttr() to a function (#106611)

* Convert PyObject_DelAttr() and PyObject_DelAttrString() macros to
  functions.
* Add PyObject_DelAttr() and PyObject_DelAttrString() functions to
  the stable ABI.
* Replace PyObject_SetAttr(obj, name, NULL) with
  PyObject_DelAttr(obj, name).

Include/abstract.h

@@ -80,7 +80,7 @@ extern "C" {
 
    This is the equivalent of the Python statement o.attr_name=v. */
 
-/* Implemented as a macro:
+/* Implemented elsewhere:
 
    int PyObject_DelAttrString(PyObject *o, const char *attr_name);
 
@@ -88,17 +88,15 @@ extern "C" {
    -1 on failure.
 
    This is the equivalent of the Python statement: del o.attr_name. */
-#define PyObject_DelAttrString(O, A) PyObject_SetAttrString((O), (A), NULL)
 
 
-/* Implemented as a macro:
+/* Implemented elsewhere:
 
    int PyObject_DelAttr(PyObject *o, PyObject *attr_name);
 
    Delete attribute named attr_name, for object o. Returns -1
    on failure.  This is the equivalent of the Python
    statement: del o.attr_name. */
-#define  PyObject_DelAttr(O, A) PyObject_SetAttr((O), (A), NULL)
 
 
 /* Implemented elsewhere:

Include/object.h

@@ -391,9 +391,11 @@ PyAPI_FUNC(PyObject *) PyObject_RichCompare(PyObject *, PyObject *, int);
 PyAPI_FUNC(int) PyObject_RichCompareBool(PyObject *, PyObject *, int);
 PyAPI_FUNC(PyObject *) PyObject_GetAttrString(PyObject *, const char *);
 PyAPI_FUNC(int) PyObject_SetAttrString(PyObject *, const char *, PyObject *);
+PyAPI_FUNC(int) PyObject_DelAttrString(PyObject *v, const char *name);
 PyAPI_FUNC(int) PyObject_HasAttrString(PyObject *, const char *);
 PyAPI_FUNC(PyObject *) PyObject_GetAttr(PyObject *, PyObject *);
 PyAPI_FUNC(int) PyObject_SetAttr(PyObject *, PyObject *, PyObject *);
+PyAPI_FUNC(int) PyObject_DelAttr(PyObject *v, PyObject *name);
 PyAPI_FUNC(int) PyObject_HasAttr(PyObject *, PyObject *);
 PyAPI_FUNC(PyObject *) PyObject_SelfIter(PyObject *);
 PyAPI_FUNC(PyObject *) PyObject_GenericGetAttr(PyObject *, PyObject *);