mypy

gh-150411: fix `gc_generation.count` race in free-threading (#150413) #21759

Job	Run time
Run mypy on Tools/check-c-api-docs	17s
Run mypy on Tools/clinic	18s
Run mypy on Lib/test/libregrtest	20s
Run mypy on Lib/tomllib	18s
Run mypy on Lib/_pyrepl	20s
Run mypy on Tools/peg_generator	22s
Run mypy on Tools/jit	18s
Run mypy on Tools/cases_generator	19s
Run mypy on Tools/build	17s
	2m 49s

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