[docs] [issue19861] Update What's New for Python 3.4

[R. David Murray]

R. David Murray added the comment:

I did not forget about set_executable...that method now works on unix *because* spawn is supported on unix now, so I don't see any need to document that separately.  (It didn't previously have an 'availability windows' line, so I think its docs need some improvement if it only applies to spawn, but that's a separate issue.)

HTTPError.headers I did forget about, thanks.

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