Restrict the allowed number of open PRs to django/django for new contributors

[Sarah Boyce]

Following discussions with many Django contributors, we have agreed that Django should use a GitHub Action to limit the number of open pull requests from new contributors.

When a pull request is opened on django/django, the action will check whether the contributor meets all three conditions:

• Has fewer than five commits on Django’s main branch.
• Has another open pull request created within the past six months.
• Is not a member of a Django organisation GitHub team, such as the triage and review team, accessibility team, or steering council.

If all three conditions hold, the new pull request will be automatically closed with an explanatory message.

Thank you for contributing to Django!

Since you have fewer than five commits in Django and already have an open pull request created within the past six months, please finish or close that pull request before opening another. We are closing this pull request because you have reached this limit.

Please focus on your existing pull request and any outstanding review feedback. If you would prefer to work on this issue, you can close your existing pull request and unassign yourself from its associated ticket so someone else can take it on. You can then reopen this pull request.

The aim is to help new contributors learn Django’s contribution and review process by focusing on one contribution at a time, responding to feedback, and bringing their work to completion. This would also discourage contributors from claiming several tickets at once, leaving them unavailable to others.

The limits that I have suggested (5 commits, 6 months etc) can be later changed or changed during review.

[Natalia Bidart]

Thank you! This matches our conversations.

[Jacob Walls]

Meets requirements and adds smart refactors. All my minor feedback is optional.

[Sarah Boyce <42296566+sarahboyce@…>]

In 2a79923:

Refs #37426 -- Moved scripts GitHub API helpers to a shared module for reuse.

[Sarah Boyce <42296566+sarahboyce@…>]

In 9b83f75:

Fixed #37426 -- Added a pull request limit for new contributors.