build: enable Python 3.15 wheels

[shaanmajid]

Update pybind11 to 3.1.0 and cibuildwheel to 4.2.1 so the existing wheel workflow builds wheels for Python 3.15. Add the Python 3.15 classifier. Build selection still includes Python 3.9.

AI assistance: I used Codex to inspect the build configuration, prepare the toolchain updates, and run local builds and tests.

[jordimas]

Both Windows wheel jobs build the Python 3.9 wheel, then fail in delvewheel repair with Unable to find library: ctranslate2.dll, even though the logs show the DLL is included in the wheel.

The cibuildwheel upgrade enables this step by default, breaking both Windows builds.

[jordimas]

Moving from pybind11 2 to 3 changes the bindings ABI and module initialization, so successful compilation alone isn’t enough.

For basic validation, we could run the existing translation and StorageView tests against the built Python 3.14/3.15 wheels. Those already cover callbacks, async results, and array ownership. A separate python -X dev -c 'import ctranslate2' check would catch shutdown allocator issues, and a small CUDA/ROCm inference test would cover GPU interoperability.