[Mokto]

Builds and tests Python 3.15 wheels. Refs #354.

• Adds cp315 and free-threaded cp315t to the Linux, macOS and Windows wheel matrices. Like 3.14, Linux wheels use manylinux_2_28 and musllinux_1_2 only.
• Upgrades cibuildwheel from 4.1.0 to 4.3.4, the first line that builds against the final CPython 3.15.0 rather than a beta, and drops the cpython-prerelease enable flag, which 3.15 no longer needs.
• Adds 3.15 and 3.15t to the test and typing matrices. The Rust backend step is skipped on 3.15 because PyO3 0.25 rejects it ("newer than PyO3's maximum supported version (3.14)").
• Adds the 3.15 classifier and updates installing.rst and news.rst.

No C, CFFI or Python changes were needed.

Relationship to #357

#357 also adds 3.15, but it depends on the PyO3 0.29 upgrade in #356 and the CI fixes in #358. This PR applies to main on its own, so 3.15 wheels can ship without waiting for the Rust backend work. It also builds against 3.15.0 final; #357 keeps cibuildwheel 4.1.0 with cpython-prerelease, which builds against a 3.15 beta. If #357 lands first, I'll close this one.

Testing

Fork CI run on this branch:

• test.yml: all 3.15 and 3.15t jobs on Linux x64/arm64, macOS arm64/x64 and Windows x86/x64/arm64 pass the C and CFFI backends, except macos-26-intel 3.15 CFFI. That job hit a hypothesis DeadlineExceeded in a fuzzing test (20.4s against a 20s deadline). The same flake hit macos-26-intel 3.14 in this run, and macos-26-intel 3.13 on main (run).
• wheel.yml: 28 of 30 cp315/cp315t jobs pass, including installing and testing the built wheel. The two failures are s390x musllinux_1_2. Every s390x musllinux_1_2 job fails the same way on main (run): the zstandard wheel builds and repairs, then installing the test dependency group fails to build ast-serialize, which mypy pulls in. The set of failing non-3.15 jobs is the same as on main.
• typing.yml, ruff, sdist and pypy pass.

Locally on macOS arm64 with ZSTD_SLOW_TESTS=1, ZSTD_WARNINGS_AS_ERRORS=1, PYTHONDEVMODE=1 and --hypothesis-profile=ci, both backends gave the same results on 3.14.7, 3.15.0 and 3.15.1t: C 292 passed, CFFI 266 passed with 26 skipped. mypy is clean on 3.15 and 3.15t.