Perhaps this constant existed at some point on some linux systems...google found one comment about it referencing posix.2.  I also found it mentioned in a specific version of sendmail, and in bash.  When it appears it seems to most often have the value 127.

I suspect the mention of this constant should just be deleted from the docs.  Google code search finds no mention of it in any sysexits.h file anywhere it has searched, so the chances are this constant will never be defined by Python.