Sort os.listdir output

See https://reproducible-builds.org/ for why this is good.

This patch was done while working on reproducible builds for openSUSE.

Modules/posixmodule.c

@@ -4259,6 +4259,7 @@ _posix_listdir(path_t *path, PyObject *list)
         Py_END_ALLOW_THREADS
     }
 
+    PyList_Sort(list);
     return list;
 }  /* end of _posix_listdir */
 #endif  /* which OS */