mypy

gh-123241: Document restrictions for `tp_traverse` implementations (g… #9917

Job	Run time
Run mypy on Lib/test/libregrtest	22s
Run mypy on Lib/tomllib	17s
Run mypy on Lib/_pyrepl	19s
Run mypy on Tools/peg_generator	16s
Run mypy on Tools/jit	20s
Run mypy on Tools/clinic	18s
Run mypy on Tools/build	21s
Run mypy on Tools/cases_generator	18s
	2m 31s

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