bpo-44321: Adds os.EX_OK for Windows (pythonGH-26559)

Author: SamuelMarks

Doc/library/os.rst

@@ -3656,9 +3656,10 @@ written in Python, such as a mail server's external command delivery program.
 
 .. data:: EX_OK
 
-   Exit code that means no error occurred.
+   Exit code that means no error occurred. May be taken from the defined value of
+   ``EXIT_SUCCESS`` on some platforms. Generally has a value of zero.
 
-   .. availability:: Unix.
+   .. availability:: Unix, Windows.
 
 
 .. data:: EX_USAGE

Modules/posixmodule.c

@@ -23,6 +23,10 @@
 #  include <pathcch.h>
 #endif
 
+#if !defined(EX_OK) && defined(EXIT_SUCCESS)
+#define EX_OK EXIT_SUCCESS
+#endif
+
 #ifdef __VXWORKS__
 #  include "pycore_bitutils.h"    // _Py_popcount32()
 #endif