ci: allow some dead code warnings on nightly

[davidhewitt]

Looks like the latest nightly has got some better dead code detection for unused trait methods. While that's very welcome, the careful job is blocking merges due to this new warning.

On the main nightly job, this is also being emitted as an error:

warning: method `downcast_into` is never used
  --> src/py_result_ext.rs:14:8
   |
13 | pub(crate) trait PyResultExt<'py>: Sealed {
   |                  ----------- method in this trait
14 |     fn downcast_into<T: PyTypeCheck>(self) -> PyResult<Bound<'py, T>>;
   |        ^^^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

I'm pretty sure we'll use that downcast_into method soon in #3703 or similar, so I'm going to just leave nightly broken in the very short term. I think the PR diff here should be sufficient to fix careful.

Will proceed straight to merge to unblock CI.

[davidhewitt]

Ah, we've also had rust 1.76 release today, so I've pushed a second commit to update the trybuild output.

Thanks to the beta clippy job we've not had any new clippy changes from the release, so updating was refreshingly easy!

[codspeed-hq]

CodSpeed Performance Report

Merging #3813 will degrade performances by 12.82%

_{Comparing davidhewitt:nightly-2024-02-08 (bcb7b88) with main (030a618)}

Summary

⚡ 3 improvements
❌ 2 regressions
✅ 74 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	davidhewitt:nightly-2024-02-08	Change
⚡	sequence_from_list	355.6 ns	300 ns	+18.52%
❌	not_a_list_via_downcast	242.8 ns	272.2 ns	-10.82%
⚡	list_via_extract	336.7 ns	281.1 ns	+19.76%
⚡	list_via_downcast	185 ns	157.2 ns	+17.67%
❌	f64_from_pyobject	377.8 ns	433.3 ns	-12.82%

[davidhewitt]

@adamreichold here's immediately a PR following #3811 where I couldn't split it out into two because stable and nightly were both broken in CI, and keeping two commits made sense 🙈