mypy

gh-142447: Fix cast warning in pycore_backoff.h (#142465) #18374

Job	Run time
Run mypy on Lib/test/libregrtest	23s
Run mypy on Tools/cases_generator	19s
Run mypy on Tools/jit	18s
Run mypy on Lib/tomllib	23s
Run mypy on Tools/build	21s
Run mypy on Tools/clinic	14s
Run mypy on Tools/peg_generator	28s
Run mypy on Lib/_pyrepl	19s
	2m 45s

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