fix: support PEP 604 structured output unions #2108

Job	Run time
checks / pre-commit	39s
checks / readme-snippets	12s
checks / test (3.12, locked, ubuntu-latest)	59s
checks / test (3.13, locked, windows-latest)	2m 42s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 3s
checks / test (3.12, locked, windows-latest)	3m 33s
checks / test (3.10, locked, windows-latest)	2m 44s
checks / test (3.10, locked, ubuntu-latest)	51s
checks / test (3.11, lowest-direct, ubuntu-latest)	56s
checks / test (3.11, locked, windows-latest)	2m 38s
checks / test (3.10, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 26s
checks / test (3.13, lowest-direct, ubuntu-latest)	54s
checks / test (3.14, locked, ubuntu-latest)	2m 21s
checks / test (3.10, lowest-direct, windows-latest)	2m 40s
checks / test (3.13, lowest-direct, windows-latest)	2m 55s
checks / test (3.11, lowest-direct, windows-latest)	2m 51s
checks / test (3.14, lowest-direct, windows-latest)	3m 41s
checks / test (3.13, locked, ubuntu-latest)	48s
checks / test (3.11, locked, ubuntu-latest)	52s
checks / test (3.12, lowest-direct, windows-latest)	3m 58s
checks / test (3.14, locked, windows-latest)	3m 13s
all-green	3s
	42m 59s

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