mypy

gh-138621: Increase test coverage for csv.DictReader and csv.Sniffer … #17871

Job	Run time
Run mypy on Lib/tomllib	17s
Run mypy on Tools/cases_generator	17s
Run mypy on Tools/build	19s
Run mypy on Tools/jit	18s
Run mypy on Tools/peg_generator	24s
Run mypy on Lib/_pyrepl	17s
Run mypy on Lib/test/libregrtest	17s
Run mypy on Tools/clinic	19s
	2m 28s

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