Define _Py_NULL as nullptr on C23 and newer (#108244)

C23 standard added nullptr constant:
https://en.wikipedia.org/wiki/C23_(C_standard_revision)

Author: vstinner

Include/pyport.h

@@ -24,9 +24,10 @@
 #define _Py_CAST(type, expr) ((type)(expr))
 
 // Static inline functions should use _Py_NULL rather than using directly NULL
-// to prevent C++ compiler warnings. On C++11 and newer, _Py_NULL is defined as
-// nullptr.
-#if defined(__cplusplus) && __cplusplus >= 201103
+// 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