mypy

gh-145850: Change some implementation details in struct.Struct (GH-14… #10276

Job	Run time
Run mypy on Tools/clinic	20s
Run mypy on Tools/check-c-api-docs	17s
Run mypy on Tools/cases_generator	18s
Run mypy on Lib/tomllib	18s
Run mypy on Lib/test/libregrtest	20s
Run mypy on Lib/_pyrepl	18s
Run mypy on Tools/peg_generator	19s
Run mypy on Tools/build	25s
Run mypy on Tools/jit	21s
	2m 56s

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