bpo-37419: Fix possible segfaults when passing large sequences to os.posix_spawn() (GH-14409)

Use Py_ssize_t instead of int for i.

Modules/posixmodule.c

@@ -5377,7 +5377,7 @@ parse_file_actions(PyObject *file_actions,
         return -1;
     }
 
-    for (int i = 0; i < PySequence_Fast_GET_SIZE(seq); ++i) {
+    for (Py_ssize_t i = 0; i < PySequence_Fast_GET_SIZE(seq); ++i) {
         file_action = PySequence_Fast_GET_ITEM(seq, i);
         Py_INCREF(file_action);
         if (!PyTuple_Check(file_action) || !PyTuple_GET_SIZE(file_action)) {