Backport workflow: maintain authorship

[nicoddemus]

Maintain commit authorship when creating the backport PR.

[antonblr]

I believe it will be fine if we limit the PR merge actions to "Squash and Merge" button only.

e.g. these PRs merge commit authorship got overridden by the mergers (I assume "Create a merge commit" button was used):

• Derive pytest.raises from AbstractContextManager #10660. (<-- this one caused backport to produce a PR with different commit author)
• Fix tests pygments 2.14.0 #10632

these are fine - the merge commit author preserved, even it was merged by another person:

• Fix entry-points declaration in the documentation example using Hatch #10725
• Add check for zero denominator in approx #10624

The following line gets the commit sha of the merged PR for backporting and depends on merge type the author will be the one who fixed the issue, or the one who merged the PR:

pytest/.github/workflows/backport.yml

Line 43 in 31ad577

git cherry-pick -x --mainline 1 ${{ github.event.pull_request.merge_commit_sha }}

[webknjaz]

#12475 implements this, I believe.