mypy

GH-143948: Explain graphlib's cycle-finding code (#143950) #19196

Job	Run time
Run mypy on Tools/build	19s
Run mypy on Tools/check-c-api-docs	17s
Run mypy on Lib/tomllib	19s
Run mypy on Tools/cases_generator	20s
Run mypy on Tools/jit	19s
Run mypy on Lib/_pyrepl	20s
Run mypy on Lib/test/libregrtest	19s
Run mypy on Tools/clinic	19s
Run mypy on Tools/peg_generator	20s
	2m 52s

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