mypy

Fix a stale link to the `math.integer.isqrt()` correctness proof (#15… #22785

Job	Run time
Run mypy on Lib/tomllib	28s
Run mypy on Tools/jit	30s
Run mypy on Tools/build	30s
Run mypy on Tools/cases_generator	32s
Run mypy on Tools/peg_generator	32s
Run mypy on Lib/_pyrepl	34s
Run mypy on Tools/clinic	33s
Run mypy on Lib/test/libregrtest	30s
Run mypy on Tools/check-c-api-docs	34s
	4m 43s

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