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

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