blis: add build-blis.yml for riscv64 wheels

[luhenry]

Adds .github/workflows/build-blis.yml, building riscv64 wheels for blis (explosion/cython-blis) 1.3.3 for cp312/cp313/cp314. Upstream publishes real per-interpreter compiled wheels for linux x86_64/aarch64, macOS and Windows, but nothing for riscv64.

Why this is a plain port

cython-blis does not run BLIS's own configure at build time. It ships a pregenerated blis/_src/make/<platform>-<arch>.jsonl per target, listing every object file's compiler/source/flags, and setup.py's ExtensionBuilder.compile_objects() replays it. get_arch_name() maps aarch64 to cortexa57, ppc64le to power9, and anything else that is not x86_64 to generic — so riscv64 selects linux-generic with no BLIS_ARCH override and no patch. That config's 297 compile specs carry only -O3 -fPIC -std=c99 -fvisibility=default (plus -funsafe-math-optimizations -ffp-contract=fast -fopenmp-simd on 45 of them), every source is .c, none live under an x86/avx/sse path, and include/linux-generic/blis.h pulls in no immintrin.h or inline asm. It is portable C throughout.

Shape

Build-from-checkout with cibuildwheel, mirroring upstream's cibuildwheel.yml narrowed to manylinux_riscv64. Upstream's [tool.cibuildwheel] table in pyproject.toml is inherited as-is (build frontend, auditwheel repair command); the only overrides are the riscv64 image and PIP_EXTRA_INDEX_URL, which the build needs so the numpy>=2.0.0,<3.0.0 build requirement resolves to our registry's numpy 2.5.2 riscv64 wheel rather than compiling from sdist. Upstream's job-level CC=clang/AR=llvm-ar env exists for the Windows leg only and never reaches the Linux container (their environment-pass is empty), so it is dropped here and the jsonl's own gcc is used.

Upstream tests wheels in a separate workflow, tests.yml: install the wheel, delete the source tree, then python -m pytest --pyargs blis -Werror. That is reproduced as CIBW_TEST_COMMAND — the suite (blis/tests/) ships inside the wheel and --pyargs resolves it from the installed package, so there is nothing for the checkout to shadow.

Free-threaded builds are excluded: upstream's [tool.cibuildwheel] skip = "cp38* cp39* cp3??t-*" rules them out and PyPI carries no cp3XXt blis wheel on any platform.

Licensing

The wheel already ships blis-1.3.3.dist-info/licenses/LICENSE, and that file explicitly covers the vendored BLIS sources — it names The University of Texas at Austin, Hewlett Packard Enterprise and AMD alongside ExplosionAI, all under 3-clause BSD. No licence patch is needed.

Local validation

quay.io/pypa/manylinux_2_39_aarch64 with BLIS_ARCH=generic forced (same Rocky-family image, exercising the exact linux-generic.jsonl path riscv64 will take): pip wheel . produced blis-1.3.3-cp313-cp313-linux_aarch64.whl containing both blis/cy...so and blis/py...so plus the LICENSE, and python -m pytest --pyargs blis -Werror from an empty cwd passed 4/4. actionlint is clean apart from the usual unknown ubuntu-24.04-riscv label.