Modernize CI: wheel runners, Windows wheels, and test matrix (drop 3.9, require 3.14)

[wbarnha]

Description

Modernizes python-package.yml: current GitHub runner images for the release wheel build, Windows wheels, and a refreshed CPython test matrix.

Release wheel build

The build_wheels matrix pinned ubuntu-20.04, whose runner image GitHub retired in April 2025 — that job can no longer be scheduled. macos-13 is also deprecating, and pypa/cibuildwheel@v2.10.1 (Sept 2022) predates Python 3.12/3.13 wheels and the newer runner images. This job only runs on release, so the breakage is latent (same class as the dropped-artifact bug fixed in #684).

• Runners: ubuntu-20.04 → ubuntu-latest; macos-13 → macos-15-intel (x86_64); macos-14 kept (arm64).
• cibuildwheel: v2.10.1 → v2.21.3.
• Windows wheels: added windows-2022 → AMD64 CPython wheels. pyproject.toml already declares Windows support and carries a [tool.cibuildwheel] config (build = "cp3*", archs = ["auto64"], before-build = "pip install Cython"); setup.py already handles win32 (links ws2_32, skips gcc-only warning flags), so the existing build config applies unchanged. The per-runner artifact name (cibw-wheels-${{ matrix.os }}) stays unique for the publish job's cibw-* download+merge.

Test matrix

• Add Python 3.14 (with and without Cython) to test-pytest. 3.13 was already present; both now run under the required ✅ Ensure the required checks passing gate. 3.14 is GA (Oct 2025), so no prerelease handling is needed.
• Drop Python 3.9: bump the PyPy lane pypy3.9 → pypy3.11 (stays non-required / continue-on-error), and remove the 3.9 formatter fallbacks from requirements/test.txt (the black/isort pins now install unconditionally since the minimum supported Python is 3.10).
• pyproject.toml: add the 3.13 and 3.14 classifiers.

The other two workflows (codeql-analysis, gh-pages) already run on ubuntu-latest with current action versions — no changes needed.

Validation notes

• YAML/TOML validated; CPython matrix resolves to ['3.10','3.11','3.12','3.13','3.14']; no 3.9 references remain in CI or requirements.
• 3.14 is now a required check, so if a runtime dependency isn't yet 3.14-ready the gate will flag it here — that's the intended signal.
• The wheel/publish jobs are gated to release events, so this PR's checks don't build wheels; the new runners and first-ever Windows build get exercised at the next release (or a manual workflow_dispatch/test tag).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.14%. Comparing base (f3019aa) to head (9ac7ab4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #690   +/-   ##
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  Coverage   94.14%   94.14%           
=======================================
  Files         104      104           
  Lines       11136    11136           
  Branches     1201     1201           
=======================================
  Hits        10484    10484           
  Misses        551      551           
  Partials      101      101           

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