black: add build-black.yml for riscv64 wheels

[luhenry]

Adds a riscv64 wheel build for black 26.5.1, published to pypi.riseproject.dev.

Ref #324

Shape

black publishes mypyc-compiled binary wheels (cpXY-cpXY, a big speedup) for common platforms but not riscv64 — the value-add here. It's a third mypyc variant on top of the ones already in the repo:

package	backend	mypyc trigger
tomli	flit_core → setuptools swap script	TOMLI_USE_MYPYC=1
mypy	setuptools	MYPY_USE_MYPYC=1
black	hatch-mypyc build hook	HATCH_BUILD_HOOKS_ENABLE=1

black ships a full [tool.cibuildwheel] config and bundles its tests, so this is the "easy inverse" (CLAUDE.md gotcha 6): we check out the tag and inherit that config wholesale — the hatch-mypyc hook, CC=clang (+ yum install -y clang), and upstream's own pytest {project} -k "not incompatible_with_mypyc" -n auto with the d/jupyter test-extras. The only override is CIBW_MANYLINUX_RISCV64_IMAGE.

Deliberately not setting CIBW_ENVIRONMENT: as an env var it replaces black's TOML [tool.cibuildwheel.environment] table rather than merging, which would silently drop HATCH_BUILD_HOOKS_ENABLE=1 and ship a pure-Python no-op. Not needed anyway — every runtime + test dep (aiohttp, ipython, …) already resolves from public PyPI.

Matrix

cp312 / cp313 / cp314 — per-interpreter (mypyc wheels aren't abi3). No free-threaded build: upstream skips it (cp31?t-*, "mypyc doesn't have great support for free threaded builds"), and PyPI confirms no cp313t/cp314t wheel.

Validation

• YAML parses; actionlint clean (only the expected ubuntu-24.04-riscv unknown-label warning).
• QEMU riscv64 (cibuildwheel --only cp312-manylinux_riscv64, aarch64 host): confirmed yum install -y clang succeeds on the image (clang 21.1.8) and the build's isolated env pulls in hatch-mypyc>=0.16.0 + mypy==1.20.2 — i.e. the compile hook is engaged, not a pure-Python fallback.
• Version: shallow tag checkout → git describe yields a clean 26.5.1 (hatch-vcs), matching the canonical PyPI filename (no local segment).

[threexc]

New issue to reference: #324

[threexc]

I don't want to make the workflows run again, but future commits could just do Ref: <issue_number> so that the issues we use have links to each relevant PR without closing/re-opening them each time.

[luhenry]

@threexc I've updated Python Wheels to remove that behavior