gmpy2: add build-gmpy2.yml for riscv64 wheels

[luhenry]

• Package: gmpy2
• Version: 2.3.1
• Source: https://github.com/gmpy2/gmpy2
• Docs: https://github.com/gmpy2/gmpy2

Compiles a CPython extension wrapping GMP, MPFR and MPC for arbitrary-precision arithmetic. Upstream publishes no riscv64 wheel.

Mirrors upstream's wheels.yml, which already builds GMP/MPFR/MPC from source per-wheel via scripts/cibw_before_all.sh and vendors them with auditwheel/delocate.

Differs from upstream

• musllinux dropped - test-extras' numpy/cython have no musllinux riscv64 wheel on PyPI or our registry.
• cp314t builds but skips tests - Cython (a test-only dep) ships no free-threaded wheel anywhere.

Testing

• same as upstream, except cp314t (see above).

License: Wheels dynamically link GMP/MPFR/MPC (LGPL-3.0-or-later) and vendor them via auditwheel/delocate; upstream ships no licence text for them, so the patch adds it.

Patches

• 0001-add-GMP-MPFR-MPC-licences-next-to-the-package-s-own.patch - Upstream-Status: To upstream [not yet submitted]. Adds LICENSE.gmp/mpfr/mpc so the LGPL notices ship in the wheel; reproduces on any arch.

Built on cp312; 367 passed, 7 skipped. cp313: 373 passed, 1 skipped. cp314: 374 passed. cp314t: tests skipped (see above).