This is not ready to merge yet! But it's the area I'm most uncomfortable with updating, so I'm getting the PR out early.

@ambv suggested that we should treat PyManager releases as another Python release, so I think that's what I've enabled. This will put them in all the same release lists as Python (as long as downloads only exist for Windows, Ibelieve it won't appear on other OS lists), and it makes updating the "Download latest" buttons a bit easier.

I'm leaving the old installer available in those popups for as long as it exists, so that we don't seem quite as pushy. The installer will have its own message warning about deprecation. Eventually I'd like to replace these messages with a "Runpy install {version}" message, but haven't quite figured that part out - it's not urgent.

Very open to any kind of help/feedback/assistance here, this is somewhat out of my depth, and while Icould spend time learning all the ins and outs, I'd rather just ask for help :)