gh-106320: Remove private _PyList functions (#108451)

Move private functions to the internal C API (pycore_list.h):

* _PyList_Extend()
* _PyList_DebugMallocStats()

No longer export these functions.

Author: vstinner

Include/cpython/listobject.h

@@ -21,9 +21,6 @@ typedef struct {
     Py_ssize_t allocated;
 } PyListObject;
 
-PyAPI_FUNC(PyObject *) _PyList_Extend(PyListObject *, PyObject *);
-PyAPI_FUNC(void) _PyList_DebugMallocStats(FILE *out);
-
 /* Cast argument to PyListObject* type. */
 #define _PyList_CAST(op) \
     (assert(PyList_Check(op)), _Py_CAST(PyListObject*, (op)))

Include/internal/pycore_list.h

@@ -8,7 +8,9 @@ extern "C" {
 #  error "this header requires Py_BUILD_CORE define"
 #endif
 
-#include "listobject.h"           // _PyList_CAST()
+
+extern PyObject* _PyList_Extend(PyListObject *, PyObject *);
+extern void _PyList_DebugMallocStats(FILE *out);
 
 
 /* runtime lifecycle */