mypy

gh-123241: Document restrictions for `tp_traverse` implementations (g… #18347

Job	Run time
Run mypy on Tools/cases_generator	19s
Run mypy on Lib/test/libregrtest	22s
Run mypy on Lib/tomllib	20s
Run mypy on Lib/_pyrepl	16s
Run mypy on Tools/clinic	20s
Run mypy on Tools/peg_generator	17s
Run mypy on Tools/jit	21s
Run mypy on Tools/build	17s
	2m 32s

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