stubgen: Use `Generator` type var defaults (#17670) #13152

Job	Run time
Test suite with Python nightly	14m 26s
Test mypyc suite with 32-bit Python	7m 3s
Test suite with py38-ubuntu, mypyc-compiled	17m 40s
Test suite with py38-windows-64	20m 47s
Test suite with py39-ubuntu	19m 45s
Test suite with py310-ubuntu	15m 46s
Test suite with py311-ubuntu, mypyc-compiled	16m 12s
Test suite with py312-ubuntu, mypyc-compiled	18m 22s
mypyc runtime tests with py39-macos	7m 33s
mypyc runtime tests with py38-debug-build-ubuntu	4m 33s
Type check our own code (py38-ubuntu)	1m 50s
Type check our own code (py38-windows-64)	2m 23s
Formatting and code style with Black + ruff	37s
	2h 26m 57s

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