datasketches: add build-datasketches.yml for riscv64 wheels

[luhenry]

• Package: datasketches
• Version: 5.2.0
• Source: https://github.com/apache/datasketches-python
• Docs: http://datasketches.apache.org

Compiles the nanobind/CMake _datasketches extension (probabilistic streaming-algorithm sketches: HLL, KLL, CPC, theta, tuple, quantiles, and friends) that CMake's ExternalProject_Add builds against a fetched checkout of apache/datasketches-cpp at the matching tag, plus the pure-Python datasketches wrapper package. Upstream publishes no riscv64 wheel.

Mirrors upstream's build_wheels.yml.

Differs from upstream

• Nothing beyond the riscv64 image and the patch below.

Testing

• tests/ staged into an empty cwd (CIBW_TEST_SOURCES) since the checkout root's own datasketches/ source package would otherwise shadow the installed wheel; same suite tox's changedir = tests runs.

License: OK

Patches

• 0001-Drop-const-...patch - To upstream. vector_of_kll_sketches's own operator= assigns to its const k_/d_ members; GCC rejects it building the nanobind bindings. Reproduces on any GCC manylinux arch, not riscv64-only.

Built on cp312 equivalent (aarch64 manylinux, local); 37 passed.