Bump required Python version to 3.9 and switch from extra dependencies to dependency groups

[dnicolodi]

No description provided.

[dnicolodi]

Dependency groups are supported starting with pip 25.1 which dropped support for Python 3.8, thus this will have to wait till we drop support for Python 3.8 or start using uv to manage the test environments.

[rgommers]

Python 3.8 has been EOL for over a year, and Python 3.9 is also EOL now (see https://devguide.python.org/versions/). So I think dropping 3.8 now is perfectly fine.

[dnicolodi]

@rgommers I opted for dropping Python 3.8 from the CI jobs but not bump the required Python version. Should we bump requires-python to >= 3.9?

[rgommers]

Should we bump requires-python to >= 3.9?

I think so, yes. 3.8 support will not go anywhere for 0.18.0, and almost no one needs it anymore so I doubt future bug fixes are going to matter. So if we're dropping CI jobs, bumping requires-python is preferred imho.

[eli-schwartz]

I think so, yes. 3.8 support will not go anywhere for 0.18.0, and almost no one needs it anymore so I doubt future bug fixes are going to matter. So if we're dropping CI jobs, bumping requires-python is preferred imho.

👍 to explicitly "breaking it" if you decline to test it. Meson does support 3.7 though I suppose most meson-python users are less concerned about backporting to LTS distros.

[rgommers]

though I suppose most meson-python users are less concerned about backporting to LTS distros.

Indeed. We have never had a case of a bug fix in the latest release being critical for an LTS distro for a Python version we dropped. In the quite unlikely case it does happen, it can be dealt with then by the distro applying a patch or by a new 0.18.1 bug fix release.

[rgommers]

All LGTM, in it goes.