Fix compatibility with pytest-run-parallel and add free-threaded CI jobs #328

Job	Run time
tests / Load tox environments	8s
tests / codestyle (ubuntu-latest)	14s
tests / py311-test-pytest72 (ubuntu-latest)	36s
tests / py310-test-pytest62 (ubuntu-latest)	38s
tests / py39-test-pytest53 (ubuntu-latest)	36s
tests / py310-test-pytest70 (macos-latest)	45s
tests / py310-test-pytest71 (windows-latest)	1m 43s
tests / py39-test-pytest60 (macos-latest)	51s
tests / py313-test-devdeps (ubuntu-latest)	1m 10s
tests / py313t-test-parallel (ubuntu-latest)	3m 45s
tests / py39-test-pytest61 (windows-latest)	1m 20s
tests / py313-test-parallel (ubuntu-latest)	39s
tests / py312-test-pytest74 (windows-latest)	1m 29s
tests / py313-test-pytest83 (ubuntu-latest)	38s
tests / py313-test-pytest90 (ubuntu-latest)	42s
tests / py39-test-pytestoldest (windows-latest)	1m 21s
tests / py311-test-pytest73 (macos-latest)	52s
tests / py313t-test (ubuntu-latest)	3m 42s
tests / report overall test coverage	-1s
publish	0s
	21m 8s

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