Add pyproject.toml and require Python 3.12 for local dev

[Odilhao]

Why are you introducing these changes? (Problem description, related links)

./setup-environment fails on CentOS Stream 9/RHEL 9/UBI9 because their default Python is 3.9, and development/requirements.txt pins pytest>=9.0, which dropped Python 3.9 support (requires 3.10+). Downgrading the pytest pin would just be papering over a version mismatch: downstream packaging already targets Python 3.12 (python-obsah builds against python3.12, foremanctl.spec requires python3.12-obsah) and CI already sets up Python 3.12 for every job. EL9's default 3.9 was never actually a supported target for this project, and EL10 ships 3.12 by default anyway.

What are the changes introduced in this pull request?

• Add pyproject.toml declaring requires-python = ">=3.12" plus project metadata and dependencies. This is metadata-only (foremanctl/forge stay as bash wrappers, no console-script entry points, no bundling of src//development/ as package data). Version is resolved via setuptools_scm from git describe, mirroring what VERSION/export-subst does for release tarballs but working correctly from a plain checkout too (a custom tag_regex handles this repo's non-PEP-440 tags like 3.0.0-develop). The build backend (setuptools) is left unpinned so it uses whatever version already ships with the running system rather than one pip would fetch from PyPI.
• setup-environment now checks for python3.12 explicitly (instead of relying on whatever python resolves to on PATH) and fails fast with a clear, actionable error if it's missing.
• src/requirements.txt / development/requirements.txt are left unchanged for now — this is a first step, not a full migration.
• DEVELOPMENT.md now lists Python - 3.12+ under Requirements.

How to test this pull request

Steps to reproduce:

• On a machine/container without python3.12 on PATH (e.g. plain quay.io/centos/centos:stream9), run ./setup-environment and confirm it fails fast with Error: python3.12 is required ... instead of pip's confusing "no matching distribution" error.
• Install python3.12/python3.12-devel (CS9) or just use CS10 (ships python3.12 as default python3), run ./setup-environment, and confirm .venv/bin/python --version reports 3.12 and pytest>=9.0 installs successfully.
• In that venv, run pip install --upgrade setuptools setuptools_scm && pip install --no-build-isolation -e '.[dev]' and confirm it builds using the system's own setuptools (check pip show setuptools matches the OS package version) and resolves a valid version via pip show foremanctl.

This was verified end-to-end with podman run on both quay.io/centos/centos:stream9 and :stream10.

Checklist

• Tests added/updated (if applicable) — verified manually via podman on CS9/CS10; no automated test suite covers setup-environment itself.
• Documentation updated (if applicable) — DEVELOPMENT.md

[jeremylenz]

setup-environment now checks for python3.12 explicitly (instead of relying on whatever python resolves to on PATH) and fails fast with a clear, actionable error if it's missing.
src/requirements.txt / development/requirements.txt are left unchanged for now — this is a first step, not a full migration.
DEVELOPMENT.md now lists Python - 3.12+ under Requirements.

So which is it? 3.12+ or 3.12 exactly?

[Odilhao]

Changed to one dynamic resolution that will failed if it's not 3.12+, when I tried to run on CS9 with only python3.9 it failed, and later worked with 3.12.

[ekohl]

./setup-environment fails on CentOS Stream 9/RHEL 9/UBI9 because their default Python is 3.9, and development/requirements.txt pins pytest>=9.0, which dropped Python 3.9 support (requires 3.10+).

I'm sorry for adding that. I didn't realize it when I bumped it for the subtests feature.

I don't like duplicating the dependencies in 2 places. IMHO that needs to be solved.

[ekohl]

Runtime wise I think this is lower. It's just for testing now.