Python 3.15 wheels for highspy #3444

What language and solver does this apply to?
Python (highspy) / MIP

Describe the problem you are trying to solve.
CPython 3.15.0 is released, but highspy 1.15.1 has no cp315 wheels on PyPI. Installs on 3.15 fall back to building the sdist, which needs a C++ toolchain and several minutes, and fails on slim container images without a compiler (e.g. python:3.15-slim).

Describe the solution you'd like
Add cp315 to the wheel matrix in .github/workflows/build-wheels-push.yml (on latest it lists cp39 to cp314 and cp314t) and publish a release with the new wheels. The workflow pins pypa/cibuildwheel@v4.0.0, which only builds a 3.15 beta behind the cpython-prerelease flag; v4.2.0 builds 3.15 by default and v4.3.1 uses 3.15.0 final, so the pin needs a bump too.

Describe alternatives you've considered
Compiling from the sdist in our Docker build stage; we'd rather stay on 3.14 until official wheels exist.

Additional context
Same request as #2779 (resolved by #2780) for 3.14.