Update _Py_NULL for C23

Author: vstinner

pythoncapi_compat.h

@@ -30,14 +30,14 @@ extern "C" {
 #  define _Py_CAST(type, expr) ((type)(expr))
 #endif
 
-// On C++11 and newer, _Py_NULL is defined as nullptr on C++11,
-// otherwise it is defined as NULL.
-#ifndef _Py_NULL
-#  if defined(__cplusplus) && __cplusplus >= 201103
-#    define _Py_NULL nullptr
-#  else
-#    define _Py_NULL NULL
-#  endif
+// Static inline functions should use _Py_NULL rather than using directly NULL
+// to prevent C++ compiler warnings. On C23 and newer and on C++11 and newer,
+// _Py_NULL is defined as nullptr.
+#if (defined (__STDC_VERSION__) && __STDC_VERSION__ > 201710L) \
+        || (defined(__cplusplus) && __cplusplus >= 201103)
+#  define _Py_NULL nullptr
+#else
+#  define _Py_NULL NULL
 #endif
 
 // Cast argument to PyObject* type.