Apply suggestion from @johnslavik

Author: johnslavik

Lib/test/test_traceback.py

@@ -529,6 +529,7 @@ def test_lost_io_open(self):
         crasher = textwrap.dedent("""\
             import io
             import traceback
+            # Trigger fallback mode
             traceback._print_exception_bltin = None
             del io.open
             raise RuntimeError("should not crash")