fix reference to type being borrowed inside #[classmethod] dunder methods

[davidhewitt]

While working on #6386 I noticed a questionable call to Py_TYPE inside macro code to create #[classmethod] receivers from instance values. The call didn't increase the refcount of the type object; I'm pretty confident that this could lead to bugs if __class__ was reassigned on the object and it caused the type object to be garbage collected.

I didn't bother writing a test because it'd be pretty painful to hit the problem conditions and it feels like the sort of thing where now it's done correctly we're unlikely to regress it.

[codspeed]

Merging this PR will not alter performance

✅ 141 untouched benchmarks
⏩ 6 skipped benchmarks¹

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_{Comparing davidhewitt:class-type (c0f835a) with main (6fb0ba8)}

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. ↩

[Icxolu]

Looks correct (and also a bit cleaner)