pytest 9.0 release planning

[bluetech]

PR #13704 proposes a breaking change, so I figure we should agree that the next release can be a major before we merge it.

pytest 8.0.0 was released on 2024-01-27, and a major after 4 minors is our usual pattern.

I also propose we drop Python 3.9 since it will become end-of-life in October (source).

I can handle the necessary preparations and the release, aiming for end of September/start of October.

[nicoddemus]

All sounds good to me @bluetech. 👍

[bluetech]

I created the 9.0.x branch, any further changes to main are now for 9.1.

I started the release PR workflow, will edit the release notes and do some testing. Planning to release on Friday, and will be available to handle regressions for a few days after that.

[bluetech]

I created milestones for 9.1 and 10.0. I moved all remaining items from 9.0 to 10.0. I also closed the 8.4 milestone.