Add a free-threaded 3.15t entry to the test matrix #223

The test matrix in .github/workflows/main.yml covers 3.14t and 3.15, but not 3.15t:

        python:
        - "3.10"
        - "3.14"
        - "3.14t"
        ...
        include:
        ...
        - python: "3.15"
          platform: ubuntu-latest

Now that 3.15.0 has shipped, free-threaded 3.15 is worth its own entry. It is not simply "3.14t but newer": 3.15 is the first release with PEP 803's abi3t stable ABI, so code paths that branch on Py_GIL_DISABLED together with a 3.15-or-newer check cannot be reached by either existing job. 3.14t is free-threaded but too old; 3.15 is new enough but has the GIL.

This showed up concretely downstream in setuptools. pypa/setuptools#5193 adds abi3t wheel tagging and reported green CI while carrying a latent failure: the vendored packaging at the time could not emit an abi3t tag at all, so the PR's supported-tag assertion could never have held on a free-threaded 3.15. Nothing caught it, because nothing ran there. (That has since been fixed by pypa/setuptools#5361, but only the missing dependency — not the missing coverage.)

A 3.15t entry alongside the existing 3.15 one would close the gap for every skeleton-based project at once, rather than each repo adding it separately.

One caution from the downstream experience: free-threaded entries here have a history of dependency trouble. setuptools still carries a commented-out 3.13t entry noting "cffi (required by PyNaCl) doesn't support free-threaded CPython 3.13t", and pyobjc had issues on 3.15 (ronaldoussoren/pyobjc#671) that made 3.15 coverage Linux-only in practice. So it may be worth confirming the job can actually be green before adding it — an added-but-red matrix entry is worse than a missing one, since it makes every PR's CI unreadable.