bpo-37129: Add os.RWF_APPEND flag for os.pwritev() (GH-20336)

diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 59ac47d..47ae7a8 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -9778,6 +9778,7 @@
 
 - RWF_DSYNC
 - RWF_SYNC
+- RWF_APPEND
 
 Using non-zero flags requires Linux 4.7 or newer.
 [clinic start generated code]*/
@@ -9785,7 +9786,7 @@
 static Py_ssize_t
 os_pwritev_impl(PyObject *module, int fd, PyObject *buffers, Py_off_t offset,
                 int flags)
-/*[clinic end generated code: output=e3dd3e9d11a6a5c7 input=803dc5ddbf0cfd3b]*/
+/*[clinic end generated code: output=e3dd3e9d11a6a5c7 input=35358c327e1a2a8e]*/
 {
     Py_ssize_t cnt;
     Py_ssize_t result;
@@ -14509,6 +14510,9 @@
 #ifdef RWF_NOWAIT
     if (PyModule_AddIntConstant(m, "RWF_NOWAIT", RWF_NOWAIT)) return -1;
 #endif
+#ifdef RWF_APPEND
+    if (PyModule_AddIntConstant(m, "RWF_APPEND", RWF_APPEND)) return -1;
+#endif
 
 /* constants for posix_spawn */
 #ifdef HAVE_POSIX_SPAWN