[docs] [issue38053] Update plistlib documentation

[hai shi]

hai shi <shihai1991 at 126.com> added the comment:

>* Remove notes about the new API being added in 3.4 since 3.4 is no longer supported
  Related bpo in issue14455

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nosy: +shihai1991

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<https://bugs.python.org/issue38053>
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