docs: Create checklist for updating files/scripts when new Python version available

[bobleesj]

Problem

Say Python 3.14 is released, parts of cookiecutter/release scripts must be manually modified. We want to be prepared and make this manual process as streamlined as possible (not resorting to memory...).

Proposed solution

Make a checklist of files that need to be modified with instructions . Maybe have it somewhere like visible like REAMDE.md in this repository.

[bobleesj]

Note to myself

bg-cookiecutter

https://github.com/Billingegroup/cookiecutter

• Modify PYTHON_DEFAULT_MAX_VERSION and PYTHON_DEFAULT_MIN_VERSION in doc/source/conf.py.

release-scripts

https://github.com/Billingegroup/release-scripts

• Modify python_versions in .github/workflows/_matrix-and-codecov-on-merge-to-main.yml
• Modify python_version in .github/workflows/_publish-docs-on-release.yml
• Modify python_version in .github/workflows/_tests-on-pr.yml
• Modify python_version in .github/workflows/_check-news-item.yml
• Modify python-version in .github/workflows/_build-pure-python-package.yml