Fix nightly and add PR template (backport #36)

[mergify]

Description

• Fixes Humble's nighly CI, using python 3.12 instead python 3.10.
• Adds a minimal PR template to remember requesting backports.

@Mergifyio backport kilted jazzy humble

Contributor Checklist

• N/A Current behavior is correctly described in the README documentation.
• Applicable backports have been included in the description.

Reviewer Checklist

• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• Check CI results: changes do not issue any warning.
• Applicable backports have been requested

---

This is an automatic backport of pull request #36 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 5da3e80 has failed:

On branch mergify/bp/humble/pr-36
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 5da3e80.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   .github/pull_request_template.md
	modified:   .github/workflows/humble-ci.yml
	modified:   .github/workflows/nightly-ci.yml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .github/workflows/reusable-ci.yml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally