[M-Colley]

CPython 3.15 was released in October and osqp has no 3.15 wheels yet, so pip install osqp on 3.15 builds from the sdist, which needs CMake and a compiler.

Changes

• build_default.yml: cibuildwheel v3.3.1 → v4.3.5. cibuildwheel builds CPython 3.15 by default from 4.2.0, and 4.3.1 or later uses the final 3.15 release.
• cibuildwheel.toml: two existing overrides gain the 3.15 builds:
  • cp315-macosx_x86_64 joins the platforms that skip the osqp.nn tests, because torch has no macOS x86_64 wheels.
  • cp315t-* joins the free-threaded override.
• cp315t is skipped for now. On free-threaded 3.15 the adjoint-derivative tests crash. OSQP allocates 13 work buffers in adjoint_derivative_qdldl with malloc() and frees them with c_free, which is PyMem_RawFree here. The fix is Allocate the adjoint derivative's QDLDL buffers with c_malloc osqp#799. Once GIT_TAG points to an OSQP release that includes it, drop cp315t-* from skip.
  • The same mismatch is latent in today's wheels. Released osqp 1.1.3 on Python 3.12 crashes in derivative_test.py under PYTHONMALLOC=debug.

Side effects of cibuildwheel 4, for review

• No cp38 wheels. cibuildwheel 4 drops Python 3.8. requires-python >= 3.8 is unchanged, so 3.8 falls back to the sdist.
• Windows repair. Windows wheels are now repaired with delvewheel by default. For osqp this vendors the MSVC C++ runtime (osqp.libs/msvcp140-<hash>.dll), because the pybind11 extension uses the C++ standard library. numpy and scipy wheels do the same, and it avoids "DLL load failed" on machines without the Visual C++ redistributable. If you'd rather not vendor it, set repair-wheel-command = "" for Windows.
• Other workflows. The CUDA, MKL and wasm workflows keep their cibuildwheel pins.

Tested locally

On Windows with cibuildwheel 4.3.5, using the configured test commands:

Build	Result
cp315-win_amd64	builds and repairs; 61 passed, 3 skipped (torch tests included)
cp314t-win_amd64	56 passed, 3 skipped
cp315t-win_amd64	crashes in derivative_test.py in two of two runs; with osqp/osqp#799 applied, 56 passed, 3 skipped

--print-build-identifiers lists cp315 for Linux (x86_64, aarch64), macOS (x86_64, arm64) and Windows (amd64, arm64), and no cp315t.

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