[WIP] Drop support for Python 3.9, add support for 3.14

[rly]

Motivation

Python 3.14 was released today, minimum testing with Python 3.9 has issues (#297), and Python 3.9 reaches EOL at the end of the month, so I wanted to see what breaks if we drop support for Python 3.9 and add support for Python 3.14.

Short answer is that it's still too soon so this is a WIP. Python 3.14 is not even available on conda-forge yet.

How to test the behavior?

Run all tests

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[rly]

Blocked because zarr-python v2 requires numcodecs<0.16 and numcodecs==0.15.1 cannot be built for Python 3.14 on MacOS. Prebuilt wheels for Python 3.14 are not available on PyPI. Building on Windows and Linux succeeds.

[rly]

Building numcodecs==0.15.1 works just fine on my local Mac arm64. Building on the GitHub Mac arm64 runner must be different in some way.