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

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I'm working my way through RISCOS code removal.  I came across this note in
Misc/HISTORY: 

- os.extsep -- a new variable needed by the RISCOS support.  It is the
  separator used by extensions, and is '.' on all platforms except
  RISCOS, where it is '/'.  There is no need to use this variable
  unless you have a masochistic desire to port your code to RISCOS.

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

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