[docs] [issue25874] Add notice that XP is not supported on Python 3.5+

[Roundup Robot]

Roundup Robot added the comment:

New changeset 5114871a3ac6 by Brett Cannon in branch '3.5':
Issue #25874: Clarify platform support in the "Using Python on Windows" doc.
https://hg.python.org/cpython/rev/5114871a3ac6

New changeset c6bd004671a7 by Brett Cannon in branch 'default':
Merge for issue #25874
https://hg.python.org/cpython/rev/c6bd004671a7

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nosy: +python-dev

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue25874>
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