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http://docs.python.org/devguide/patch.html should mention that What's New should also be considered for updating if a change is either significant or requires potential modification of existing code.

This is more the duty of the committer IMO.

Yes, but that doesn't mean it shouldn't be written down in the devguide (with appropriate note) that committers need to be thinking about this. Start good habits early. =)

Then it should probably go in http://docs.python.org/devguide/committing.html 

The "What's new" is already mentioned briefly in http://docs.python.org/devguide/committing.html#news-entries. Would it be enough to rephrase that part and clarify that committers are expected to update it?
Another option is to add a new item to make patchcheck (and to http://docs.python.org/devguide/committing.html#patch-checklist), but I'm not sure it's worth doing it.

Yep, I think expanding that would be fine.

Patch attached.

"significative" -> "significant"
I would also toss in something like "or not backwards-compatible" so people know they should mention anything that will require code change to stay backwards-compatible.

Patch updated.

LGTM

New changeset 849b2fd4b06a by Ezio Melotti in branch 'default':
#17637: mention "What's New" updating guidelines in the devguide.
http://hg.python.org/devguide/rev/849b2fd4b06a