text: Title: Explicitly mention PythonItemExporter output changes in 2.11
  Body: We removed the binary mode of `PythonItemExporter` in 2.11 but it was the default one, so we should have mentioned a backwards incompatible change which is "`PythonItemExporter` used the binary output by defauilt but it no longer does" in the "Backward-incompatible changes" entry for 2.11.