Maybe 255 is a reasonable choice. Another option is some value slightly less than 126, since Posix shells have special meanings for 126, 127, and > 128 values.

My patches already document the exit status used under the Py_Exit() function: “If :c:func:`Py_FinalizeEx` indicates an error, the exit status is set to 1 [or 255].” Do you think it should be documented somewhere else as well, perhaps for the interpreter as a whole?