deprecate unsound buffer apis

[Person-93]

Mark slice accessors of PyBuffer as deprecated. See #6267

[davidhewitt]

I think we should probably add an entry to the migration guide and make the deprecation message point at alternatives, so that users can have some guidance how to migrate.

[Person-93]

The alternatives are added in #6382, I can add the migration guide after either this or that PR is merged, or as a separate PR.

[davidhewitt]

Sorry stated merging the wrong pr

[codspeed]

Merging this PR will regress 2 benchmarks

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 2 improved benchmarks
❌ 2 regressed benchmarks
✅ 137 untouched benchmarks
⏩ 6 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	extract_biguint_small	1.4 µs	1.5 µs	-10.78%
❌	extract_biguint_zero	1.4 µs	1.6 µs	-10.41%
⚡	into_biguint_small	1.5 µs	1.3 µs	+16.79%
⚡	test_empty_class_init	31.6 µs	28.7 µs	+10.13%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing Person-93:deprecate-unsound-buffer-api (a989173) with main (5f3ea66)}

Footnotes

1. 6 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Person-93]

@davidhewitt, do you think the deprecation can be included in 0.29.3 ? Maybe the migration guide should be a separate PR that gets included in 0.30 when the functions are actually removed.