Fix [ #489673 ] memory leak in test_symtable: Free the st_future slot.

The st_future slot of the symtable is not freed by PySymtable_Free()
because it is shared by the symtable and compiling structs in
compiel.c.  Since it is shared, it is explicitly deallocated when the
compiling struct is freed.

Modules/symtablemodule.c

@@ -32,6 +32,7 @@ symtable_symtable(PyObject *self, PyObject *args)
 	if (st == NULL)
 		return NULL;
 	t = Py_BuildValue("O", st->st_symbols);
+	PyMem_Free((void *)st->st_future);
 	PySymtable_Free(st);
 	return t;
 }