Build and test Python 3.15 wheels with cibuildwheel 4.3.0

[Mokto]

Summary

Adds CPython 3.15 to the wheel build matrix so wheel-only installs work on Python 3.15. The 4.0.0 release has no published 3.15-compatible wheels, which blocks --only-binary :all: installs. Publishing the new wheels needs a maintainer release.

Changes

• Upgrade pypa/cibuildwheel from 3.3.1 to 4.3.0, which supports the cp315 identifier.
• Add cp315-* to the wheel build list.
• Run the existing unittest suite against each built wheel via test-command.
• No runtime code, dependency, or version changes. Python >=3.10 and the cp310 to cp314 builds are unchanged, as are the Linux (x86_64/aarch64, manylinux/musllinux), macOS (x86_64/arm64/universal2) and Windows (AMD64) targets.

Review focus

Check the test-command path and the cibuildwheel major version bump. Other platform builds and upstream CI are still pending.

Verification so far: all identifiers, including cp315 and universal2, were confirmed with cibuildwheel 4.3.0. A local macOS arm64 wheel was installed with --no-index --only-binary :all: and passed all 51 tests on Python 3.15.0 final and 3.14.7, run from /tmp against the installed site-packages. actionlint and the diff check pass.