ci: add Python 3.14 test environment to CI matrix

[mvanhorn]

Summary

Adds a Python 3.14 test environment to CI so xarray starts exercising the latest Python alongside 3.13.

Why

Issue #11241 asks for CI to start covering Python 3.14, which has been out for several months and is now supported by numba and the rest of the core stack. Without a 3.14 environment, xarray has no signal on whether it works on the newest interpreter that users are already adopting.

Description

This begins migrating CI toward Python 3.14, which issue #11241 asks for now that 3.14 has been out for several months and numba supports it. pixi.toml gains a py314 feature pinned to python = "3.14.*" and a test-py314 environment mirroring the existing test-py313 definition, and .github/workflows/ci.yaml adds test-py314 to the pixi-env matrix. The change is additive: 3.13 stays in place so maintainers can decide the removal cadence, keeping this low-risk.

Checklist

• Closes Add python 3.14 env to CI #11241
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Claude

[dcherian]

Let's replace 3.13 with 3.14 so we minimize how much CI we run

[mvanhorn]

Done in 1e78a4d - 3.13 is replaced by 3.14 in the main matrix and the no-numba/no-dask/flaky/typing variants now run on 3.14 only. Kept the test-py313 pixi envs since ci-additional.yaml and hypothesis.yaml still reference them.

[This is Claude Code on behalf of Matt Van Horn]

[dcherian]

Should be deleting the 3.13 entries here

[keewis]

maybe use python-gil here, to be fully explicit that we're not checking the freethreading build?

See https://anaconda.org/channels/conda-forge/packages/python/overview