add 3.14t (free threading) to matrix

[paultiq]

Free Threading is added to https://docs.python.org/3.14/whatsnew/3.14.html#whatsnew314-pep779

Per #431, 3.14 was added to the test matrix... but not the free threading "3.14t" build.

This is a minimal change to check.yaml to increase the matrix.

I didn't see any new errors so far, but will experiment / possibly propose new test cases in a separate PR.

[paultiq]

Interesting, a test failure did occur on the last run, on the 3.14t Ubuntu case.

I didn't see this on run (on my repo against the full matrix), but that's the nature of threading issues.

Edit: I haven't been able to reproduce locally, or by rerunning actions.

This does suggest a deeper issue worth investigating -

tmp_path = PosixPath('/tmp/pytest-of-runner/pytest-0/test_thrashing_with_thread_poo4')
lock_type = <class 'filelock._unix.UnixFileLock'>

    @pytest.mark.parametrize("lock_type", [FileLock, SoftFileLock])
    def test_thrashing_with_thread_pool_lock_recreated_in_each_thread(
        tmp_path: Path,
        lock_type: type[BaseFileLock],
    ) -> None:
        def mess_with_file() -> None:
            with lock_type(lock_file):
                _check_file_read_write(txt_file)
    
        lock_file, txt_file = tmp_path / "test.txt.lock", tmp_path / "test.txt"
        with ThreadPoolExecutor() as executor:
            results = [executor.submit(mess_with_file) for _ in range(100)]
    
>       assert all(r.result() is None for r in results)

=========================== short test summary info ============================
FAILED tests/test_filelock.py::test_thrashing_with_thread_pool_lock_recreated_in_each_thread[UnixFileLock] - IsADirectoryError: [Errno 21] Is a directory: '.'