docs: update changelog

[henryiii]

Description

Updates the changelog with the marked PRs.

[rwgk]

The flake is #2995 again.
I think it's very important to merge #2995 before the 2.7 release.

[henryiii]

But on PyPy macOS this time?

[rwgk]

This looks great, my main concern is that #2995 isn't included.

I think we need to be completely open about the undefined behavior, to not mislead users. I'm also thinking that we are more likely to attract a volunteer to develop a fix if we're open about the issue.

To add my own experience, it is no exaggeration that I scrolled through failed jobs 100+ times to be sure it's that flake, and it's an error each time I run TSAN.

I'd be happy quickly address suggestions on #2995.

[henryiii]

This is not the final changelog before 2.7 (though I expect we are getting close), just an update to the current changelog that has been long delayed.