ci: fix Callable[] type hint on 3.8

[davidhewitt]

Callable from abc does not support [] subscripting with type vars until Python 3.9. I wrapped the type hint in quotes to make this pass on Python 3.8.

[codspeed-hq]

Merging this PR will improve performance by 13.8%

⚡ 1 improved benchmark
✅ 104 untouched benchmarks
⏩ 1 skipped benchmark¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	bench_pyclass_create	4.6 µs	4 µs	+13.8%

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_{Comparing davidhewitt:3.8-wrap-pyfunction (5c828e5) with main (9c734a1)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩