[Python-3000] os.extsep & RISCOS support removal

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    >> If RISCOS is going away should os.extsep as well?

    Guido> Yes please. It just causes more code for no good reason.

Good.  I already removed it in my sandbox. ;-)

While I'm thinking about it, should I be identifying tasks I'm working on
somewhere to avoid duplication of effort?  There was something on the
py3kstruni wiki page where people marked the failing tests they were working
on.  I'm not aware of a similar page for more general tasks.

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