sync unicodeobject FFI definitions to 3.15

[davidhewitt]

Followup to #5965

This synchronises the unicode headers to be up to date with the Python 3.15 definitions.

[codspeed-hq]

Merging this PR will not alter performance

✅ 125 untouched benchmarks

---

_{Comparing davidhewitt:unicode-apis (0aa5d68) with main (d818aef)}

[ngoldbaum]

@davidhewitt over in the maturin CI, I just hit a PyPy failure that I think is related to this:

https://github.com/PyO3/maturin/actions/runs/26537638016/job/78170726414?pr=3113#step:18:1074

    Traceback (most recent call last):
      File "/Users/runner/work/maturin/maturin/test-crates/pyo3-stub-generation/check_installed/check_installed.py", line 4, in <module>
        import pyo3_stub_generation
      File "/Users/runner/work/maturin/maturin/test-crates/venvs/integration-pyo3-stub-generation-zig-py3-pypy/lib/pypy3.11/site-packages/pyo3_stub_generation/__init__.py", line 1, in <module>
        from .pyo3_stub_generation import *
    ImportError: dlopen(/Users/runner/work/maturin/maturin/test-crates/venvs/integration-pyo3-stub-generation-zig-py3-pypy/lib/pypy3.11/site-packages/pyo3_stub_generation/pyo3_stub_generation.pypy311-pp73-darwin.so, 0x0006): symbol not found in flat namespace '__PyUnicode_ToDecimalDigit'

Not sure offhand what's broken and why this didn't show up in the PyO3 CI.