update to PyO3 0.21 beta

[davidhewitt]

Change Summary

This PR updates to PyO3 0.21 beta (and jiter 0.0.7).

It does just the first step of the PyO3 migration which advises to switch on the "gil-refs" feature and solve the rest of the updating. Hence, I've done that here, and I'll use #1085 to merge the follow up which should unlock the real performance boosts.

Related issue number

N/A

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1222 will improve performances by 20.33%

_{Comparing dh/pyo3-0.21-beta (a24b08e) with main (4a533aa)}

Summary

⚡ 22 improvements
✅ 126 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	dh/pyo3-0.21-beta	Change
⚡	test_core_future	47.5 µs	42.3 µs	+12.33%
⚡	test_core_str	31.9 µs	28.5 µs	+12.06%
⚡	test_core_future	30.1 µs	25.8 µs	+16.76%
⚡	test_date_from_datetime	39.2 µs	34.1 µs	+15%
⚡	test_date_from_datetime_str	30.1 µs	26.7 µs	+12.68%
⚡	test_date_from_str	26.9 µs	23.4 µs	+15.01%
⚡	test_core_root_model	95.1 µs	82.4 µs	+15.51%
⚡	test_definition_in_tree	4.5 ms	4 ms	+12.42%
⚡	test_generator_rust	32.7 µs	28.7 µs	+14.01%
⚡	test_list_of_nullable_core	491.6 µs	408.5 µs	+20.33%
⚡	test_small_class_core_dict	31.9 µs	28.5 µs	+12.22%
⚡	test_tuple_many_positional	34 µs	30.1 µs	+13.12%
⚡	test_tuple_many_variable	34 µs	30.1 µs	+13.23%
⚡	test_validate_literal[json-few_str_enum]	32.8 µs	28.4 µs	+15.59%
⚡	test_validate_literal[python-few_mixed]	26.9 µs	23.7 µs	+13.49%
⚡	test_validate_literal[python-few_str_enum]	25 µs	21.1 µs	+18.35%
⚡	test_with_default	41.8 µs	37.2 µs	+12.54%
⚡	test_core_model_json	50.2 µs	44.2 µs	+13.76%
⚡	test_dataclass_serialization_json	44.9 µs	40 µs	+12.19%
⚡	test_dataclass_serialization_python	43.1 µs	38.4 µs	+12.02%
...	...	...	...	...

ℹ️ Only the first 20 benchmarks are displayed. Go to the app to view all benchmarks.

[davidhewitt]

@samuelcolvin has given me blessing to merge that, so here we go!