pythongh-108765: Python.h no longer includes <stddef.h>

In practice, only Windows is impacted, since the HAVE_STDDEF_H macro
was only defined on this platform.

Doc/whatsnew/3.13.rst

@@ -1186,6 +1186,15 @@ Porting to Python 3.13
   ``PyUnicode_AsUTF8AndSize(unicode, NULL)`` can be used instead.
   (Contributed by Victor Stinner in :gh:`111089`.)
 
+* On Windows, ``Python.h`` no longer includes the ``<stddef.h>`` standard
+  header file. If needed, it should now be included explicitly. For example, it
+  provides ``offsetof()`` function, and ``size_t`` and ``ptrdiff_t`` types. In
+  practice, it should only impact a minority of users, because including
+  ``<stddef.h>`` explicitly was already needed by all other platforms. The
+  ``HAVE_STDDEF_H`` macro is only defined on Windows.
+  (Contributed by Victor Stinner in :gh:`108765`.)
+
+
 Deprecated
 ----------
 

Include/Python.h

@@ -22,9 +22,6 @@
 #include <math.h>                 // HUGE_VAL
 #include <stdarg.h>               // va_list
 #include <wchar.h>                // wchar_t
-#ifdef HAVE_STDDEF_H
-#  include <stddef.h>             // size_t
-#endif
 #ifdef HAVE_SYS_TYPES_H
 #  include <sys/types.h>          // ssize_t
 #endif

Misc/NEWS.d/next/C API/2023-10-31-18-22-03.gh-issue-108765._beYv8.rst

@@ -0,0 +1,3 @@
+On Windows, ``Python.h`` no longer includes the ``<stddef.h>`` standard
+header file. If needed, it should now be included explicitly. Patch by
+Victor Stinner.