announcement: python3 readiness and howto

Merged Georges Racinet requested to merge topic/default/py3-0.13.0rc2 into branch/default

Announce Python 3 to the world, encourage people to try it, and tell them how.

I've tailored the first paragraph after the screenshot so that it makes sense on the category listing (where you land by clicking "announcements" in the menu).

Edited by Georges Racinet

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