stubgen: use PEP 604 unions everywhere #11629

Job	Run time
Test mypyc suite with 32-bit Python	8m 19s
Test suite with py38-ubuntu, mypyc-compiled	19m 47s
Test suite with py38-windows-64	26m 42s
Test suite with py39-ubuntu	20m 33s
Test suite with py310-ubuntu	18m 59s
Test suite with py311-ubuntu, mypyc-compiled	18m 3s
Test suite with py312-ubuntu, mypyc-compiled	21m 45s
mypyc runtime tests with py39-macos	7m 38s
mypyc runtime tests with py38-debug-build-ubuntu	5m 31s
Type check our own code (py38-ubuntu)	1m 49s
Type check our own code (py38-windows-64)	2m 30s
Formatting and code style with Black + ruff	53s
	2h 32m 29s

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