deprecate Bound|Py::(try_)borrow(_mut) (Part 2)

[Icxolu]

Part 2 of #6120

Deprecates (try_)borrow(_mut) family of methods on Bound and Py, and introducing try_borrow_guard and try_borrow_guard_mut as successors (to be renamed back at some later release). I intentionally did not reintroduce panicking variants. I think the error condition got more likely now with free-threading and we should encourage users to handle such errors gracefully.

Only the second commit is relevant, first commit is #6120. I'll rebase once #6120 lands.

• Write migration guide entry

See also #6083

[codspeed-hq]

Merging this PR will improve performance by 10.55%

⚡ 1 improved benchmark
✅ 140 untouched benchmarks
⏩ 6 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_empty_class_init	31.6 µs	28.6 µs	+10.55%

Tip

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_{Comparing Icxolu:deprecate/borrow (072ece1) with main (ee629fb)}

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. ↩