Fix stub generation false reporting #13110

Job	Run time
Test mypyc suite with 32-bit Python	6m 48s
Test suite with Python nightly	13m 57s
Test suite with py38-ubuntu, mypyc-compiled	17m 25s
Test suite with py38-windows-64	19m 41s
Test suite with py39-ubuntu	16m 20s
Test suite with py310-ubuntu	15m 45s
Test suite with py311-ubuntu, mypyc-compiled	25m 7s
Test suite with py312-ubuntu, mypyc-compiled	17m 42s
mypyc runtime tests with py39-macos	4m 24s
mypyc runtime tests with py38-debug-build-ubuntu	4m 27s
Type check our own code (py38-ubuntu)	1m 50s
Type check our own code (py38-windows-64)	2m 27s
Formatting and code style with Black + ruff	38s
	2h 26m 31s

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