expose O_PATH if possible

Doc/library/os.rst

@@ -1137,6 +1137,7 @@ or `the MSDN <http://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx>`_ on Window
           O_DIRECTORY
           O_NOFOLLOW
           O_NOATIME
+          O_PATH
 
    These constants are GNU extensions and not present if they are not defined by
    the C library.

Misc/NEWS

@@ -277,6 +277,8 @@ Core and Builtins
 Library
 -------
 
+- Expose the O_PATH constant in the os module if it is available.
+
 - Issue #17368: Fix an off-by-one error in the Python JSON decoder that caused
   a failure while decoding empty object literals when object_pairs_hook was
   specified.

Modules/posixmodule.c

@@ -10858,6 +10858,9 @@ all_ins(PyObject *d)
 #ifdef O_SEARCH
     if (ins(d, "O_SEARCH", (long)O_SEARCH)) return -1;
 #endif
+#ifdef O_PATH
+    if (ins(d, "O_PATH", (long)O_PATH)) return -1;
+#endif
 #ifdef O_TTY_INIT
     if (ins(d, "O_TTY_INIT", (long)O_TTY_INIT)) return -1;
 #endif