stubgen: Use `Generator` type var defaults (#17670) #448

Job	Run time
Test mypyc suite with 32-bit Python	6m 48s
Test suite with Python nightly	14m 31s
Test suite with py38-ubuntu, mypyc-compiled	18m 21s
Test suite with py38-windows-64	20m 3s
Test suite with py39-ubuntu	19m 45s
Test suite with py310-ubuntu	15m 44s
Test suite with py311-ubuntu, mypyc-compiled	15m 45s
Test suite with py312-ubuntu, mypyc-compiled	17m 45s
mypyc runtime tests with py39-macos	4m 41s
mypyc runtime tests with py38-debug-build-ubuntu	4m 21s
Type check our own code (py38-ubuntu)	1m 55s
Type check our own code (py38-windows-64)	2m 31s
Formatting and code style with Black + ruff	41s
	2h 22m 51s

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