Automate cherry-picking workflow

[mossheim]

Motivation

related to #5129 , automating cherry-picking would save maintainers time, reduce confusion, and simplify the changelog and release processes.

Description of Proposed Feature

i think i've come up with a reasonable workflow. we'd adopt the following workflow:

1. humans would label PRs with a special and temporary "to-cherry-pick" label at any point
2. a python script in tools could be run by any maintainer at any time to generate a cherry-pick PR; permissions to add/remove labels would be necessary

using labels instead of a project board is mainly a safety step -- right now when you add a PR to a project board by default it goes into a special 'awaiting triage' state, and programmatically querying about the project won't return that PR. there's a risk of someone adding the PR to the board and it never gets noticed or gets noticed too late. presence of a label is a binary state, so there's no such risk.

Plan for Implementation

the script would do the following:

1. filter the set of PRs for those that are (1) merged, and (2) labelled "to-cherry-pick" (optionally check that each of these PRs is against develop)
2. get the associated commits from each PR (see farther down)
3. sort the PRs in commit order (this will lead to fewer merge conflicts)
4. cherrypick the contents of each PR onto a new branch
5. make a PR to github with the new branch
6. remove the "to-cherry-pick" label from the source PRs

the title and/or text of the PR should be something easily parsed by a script so that we can use it in #5129 .

we should probably also add an 'undo' script that closes the PR and adds back the labels, in case something goes wrong or one of the PRs shouldn't be cherry-picked.

EDIT: forgot to add my notes about how to get associated commits from a PR. i couldn't find a way to do it more easily than this:

• if the merge commit has 1 parent, use just the merge commit (it was a squash)
• if the merge commit has 2 parents, use the commits between it and its first parent, excluding the merge commit itself. this means selecting only "A" and "B" in the following mock commit graph:

* Merge 'topic/example' into 'develop'
|\
| * A
| * B
|/
* Merge 'topic/some-other-branch' into 'develop'

[dyfer]

I'm pinning the issue for now, since we haven't fully addressed it for the 3.12 release