idlelib/News3.txt for 3.15.0 and backports

This issue starts after the first IDLE merge that does not go into 3.14.0 and News3.text is updated for 3.15 with that item.
The issue ends after the last IDLE merge for 3.15.
News items after the first are normally added issue by issue, but they sometimes get added in separate PRs.

Branch 'main' became 3.15.0a0 as of 3.14.0 beta 1: 2025-05-07
However, unless an patch is not backported to 3.14.b# or .rc#,
idlelib/News3.txt items continued going under
What's New in IDLE 3.14.0 on both main and 3.14, until 3.14.0, 2024-10-07
Subsequent news items go under
What's New in IDLE 3.15.0 (new header) on main branch
What's New in IDLE 3.14.z (new header) on 3.14 branch
What's New in IDLE 3.13.z (existing header) on 3.13 branch
In other words, idlelib News3.txt is handled as if main were branched off as of about 3.x.0rc?.
This is different from the changelog attached to What's New in 3.x.

Release peps -- needed for proposed and actual release dates.
3.13 PEP-719
3.14 PEP-745
3.15 PEP-790

Continues #129876

Linked PRs

• gh-144200
• gh-144201
• gh-144202
• gh-152839
• gh-152842
• gh-152843
• gh-152844