mypy

gh-101100: Fix sphinx reference warnings around I/O (#139592) #19881

Job	Run time
Run mypy on Tools/cases_generator	15s
Run mypy on Lib/_pyrepl	18s
Run mypy on Lib/tomllib	16s
Run mypy on Tools/build	18s
Run mypy on Lib/test/libregrtest	18s
Run mypy on Tools/check-c-api-docs	20s
Run mypy on Tools/clinic	21s
Run mypy on Tools/peg_generator	31s
Run mypy on Tools/jit	18s
	2m 55s

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