Test and build wheels on Python 3.15

[wbarnha]

Adds Python 3.15 to CI, covering the same ground the 3.14 rows get.

Changes

• test.yml — new 3.15 and 3.15t rows (default and free-threaded builds).
• test-system-uchardet.yml — new 3.15t row. A system build compiles against upstream uchardet's headers instead of the vendored sources, so it's where a new interpreter is most likely to break first.
• build-and-upload-to-pypi.yml — cibuildwheel 4.1.0 → 4.2.0, the first release that knows about CPython 3.15. It builds and tests cp315/cp315t by default: cibuildwheel gates a Python behind the cpython-prerelease enable group only until its first release candidate, since the ABI is frozen at rc1, so no opt-in is needed and the wheels are forward-compatible with 3.15.0 final. Confirmed with --print-build-identifiers — under the old 4.1.0 pin 3.15 could not be built at any setting; cp313t remains unbuildable at 4.2.0, so the note about 3.13t being covered in test.yml still holds.
• tox.ini — the envlist was still py36–py311, predating both the 3.10 floor in requires-python and the versions CI actually runs. Now mirrors the workflow matrix.
• pyproject.toml — added the Programming Language :: Python :: 3.15 classifier (verified it's a valid trove classifier, so PyPI won't reject an upload).

allow-prereleases: true on setup-python resolves 3.15 to whatever pre-release the runner image publishes — 3.15.0rc1 today — and will pick up 3.15.0 final with no further change. It leaves the stable rows alone: it widens 3.X to ~3.X.0-0, and a pre-release only wins when no stable release satisfies the spec. Verified against the real actions/python-versions manifest: 3.10 → 3.10.20, 3.12 → 3.12.13, 3.14 → 3.14.7, identical with and without the flag; without it 3.15 resolves to nothing, which is the "Version 3.15 was not found in the local cache" failure.

The new rows are advisory (continue-on-error) while 3.15 is a pre-release, so a CPython-side regression in rc2 or final can't block merges. The workflow comments record how to promote them.

Verification

No source change was needed. Ran against the actual 3.15.0rc1 build the runners use:

	result
3.15.0rc1	extension compiles, 150 passed, 3 skipped
3.15.0rc1 free-threaded (3.15t)	151 passed, 2 skipped — GIL confirmed disabled

Of the three repos in this set, this is the only one that needed no code change to pass on 3.15.

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