pyproject.toml: build only cp39 wheels per platform in cibuildwheel

[o-murphy]

Summary

Restricts cibuildwheel to a single CPython target (cp39-*) per platform/arch, instead of letting it iterate over the full default matrix (cp39...cp314, including free-threaded cp313t/cp314t).

libusb_bdist_wheel.get_tag() in setup.py already forces every wheel's tag to py3-none-<plat>, since the bundled extension never touches the CPython C API. That makes the resulting wheel compatible with any CPython 3.9+ interpreter (python_requires = >=3.9 in setup.cfg), free-threaded builds included — so building it once per platform/arch is enough.

Why

Without an explicit build selector, cibuildwheel provisions and invokes a build for every CPython target in its matrix, which is unnecessary work and makes the CI logs/matrix noisier than the actual output warrants.

Closes: #38

Verification

Ran cibuildwheel locally, scoped to a single platform, with both a regular and a free-threaded target explicitly listed:

CIBW_BUILD="cp39-manylinux_x86_64 cp314t-manylinux_x86_64" CIBW_ARCHS_LINUX=x86_64 cibuildwheel --platform linux

• cp39-manylinux_x86_64 built normally (~20s).
• cp314t-manylinux_x86_64 was skipped in ~0.1s: cibuildwheel detected the py3-none-manylinux2014_x86_64... wheel already produced by the cp39 build satisfies it (Found previously built wheel ..., that's compatible with cp314t-manylinux_x86_64. Skipping build step...).

No filename collision, no build failure, and no separate artifact for the free-threaded target — confirming cp39-* alone is sufficient and there's no need to list cp314t-* (or any other version) explicitly.

Full CI run on the fork with the reduced matrix, across Linux/macOS/Windows: https://github.com/o-murphy/libusb-pkg/actions/runs/29026824766

Corresponding pull_request-triggered run on this PR: https://github.com/pyocd/libusb-package/actions/runs/29027101609

Test plan

• CI (build-wheels.yml) builds successfully on Linux/macOS/Windows with the reduced matrix
• Published wheel per platform/arch is still tagged py3-none-<plat> and installs correctly on both a regular and a free-threaded (3.13t/3.14t) interpreter

Verified the second item locally with uv: installed the cp39-built wheel from wheelhouse/ into both a CPython 3.12 venv and a genuine free-threaded CPython 3.14.3 venv (sys._is_gil_enabled() == False), then loaded the bundled libusb-1.0.so via ctypes.CDLL and called libusb_get_version() in both — got 1.0.30.12037 in both cases. Confirms the wheel isn't just tag-compatible on paper but actually installs and the native library loads and runs on a free-threaded interpreter it was never built with.

[o-murphy]

cibuildwheel: before vs. after build = "cp39-*"

Platform	Before (build identifiers)	Before (time)	After (build identifiers)	After (time)
ubuntu	28	11 min	4	10 min
windows	14	8 min	2	2 min
macos	14	2 min	2	1 min
Total	56	—	8	—

Before: run 28427229151 (last release)
After: run 29027101609 (this PR)

Published wheels (identical in both runs)

ubuntu-latest

• libusb_package-*-py3-none-manylinux2014_x86_64.manylinux_2_17_x86_64.manylinux_2_28_x86_64.whl
• libusb_package-*-py3-none-manylinux2014_aarch64.manylinux_2_17_aarch64.manylinux_2_28_aarch64.whl
• libusb_package-*-py3-none-musllinux_1_2_x86_64.whl
• libusb_package-*-py3-none-musllinux_1_2_aarch64.whl

windows-latest

• libusb_package-*-py3-none-win_amd64.whl
• libusb_package-*-py3-none-win32.whl

macos-latest

• libusb_package-*-py3-none-macosx_11_0_arm64.whl
• libusb_package-*-py3-none-macosx_10_9_x86_64.whl

Why the wheel count didn't change

The number of published wheels is set by the number of unique plat tags (manylinux_x86_64, manylinux_aarch64, musllinux_x86_64, musllinux_aarch64, win_amd64, win32, macosx_x86_64, macosx_arm64) — determined by the archs config, which this change doesn't touch. Hence 8 wheels either way.

The number of build identifiers (cp39-manylinux_x86_64, cp310-manylinux_x86_64, ..., cp314t-manylinux_x86_64, etc.) is how many times cibuildwheel attempts a build per plat. Without a build selector, it iterates over every CPython version for each plat (hence 28 on ubuntu = 4 plats × 7 versions).

Even before this fix, cibuildwheel never actually published duplicates: before each build it checks whether a wheel already in the output directory is tag-compatible with the current identifier. Since the tag is always py3-none-<plat> (compatible with any CPython version), every identifier after the first prints "Skipping build step" and produces nothing new. That's exactly what we confirmed locally with cp314t.

So: before, 28 identifiers on ubuntu → the first (cp39) actually builds, the remaining 24 are instantly skipped, still ending up with 4 files.
After, the identifier list only has 4 entries (one per plat) → all of them actually build, nothing to skip.

What changed isn't the output — it's the path to it. The redundant "check-then-skip" cycles (interpreter provisioning + compatibility check) for ~48 versions that never contributed anything are gone. This was most visible on Windows (8 min → 2 min), where provisioning each Python version was the most expensive part.

[RobertRostohar]

Thank you for providing such a detailed description, solution and testing it. Looks good to me.