mypy

gh-148947: dataclasses: fix error on empty class cell (#148948) #20875

Job	Run time
Run mypy on Tools/check-c-api-docs	15s
Run mypy on Tools/cases_generator	14s
Run mypy on Lib/tomllib	16s
Run mypy on Tools/build	19s
Run mypy on Lib/test/libregrtest	26s
Run mypy on Lib/_pyrepl	16s
Run mypy on Tools/jit	18s
Run mypy on Tools/peg_generator	16s
Run mypy on Tools/clinic	23s
	2m 43s

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