pyreadstat: add build-pyreadstat.yml for riscv64 wheels

[luhenry]

• Package: pyreadstat
• Version: 1.3.6
• Source: https://github.com/Roche/pyreadstat
• Docs: https://github.com/Roche/pyreadstat

Compiles a Cython wrapper around the vendored ReadStat C library (reads/writes SAS, SPSS and Stata files into pandas/polars). Upstream publishes no riscv64 wheel.

Mirrors the test job of upstream's ci.yml (upstream has no separate wheel-building workflow, only cibuildwheel-shaped CI tests).

Differs from upstream

• carries upstream's CFLAGS=-O2 -fno-strict-aliasing - works around a gcc 13+ miscompile of readstat_writer.c's strict-aliasing violations
• test_narwhalified.py dropped - unconditionally imports polars, which has no riscv64 wheel
• polars-only cases in test_runtime_types.py/test_typing.yml deselected for the same reason

Matrix: cp310-cp314, no cp314t - matches upstream's own PyPI wheel matrix; the registry also has no free-threaded pandas wheel

Testing

• same as upstream, minus the polars cases above

License: OK - Apache-2.0 wrapper, vendored ReadStat is MIT; both texts ship in the wheel's LICENSE file already.

Built on cp312 locally (macOS); wheel imports and the compiled extensions load correctly.