mypy

gh-123138: Updated email.headerregistry docs to include required keyw… #21646

Job	Run time
Run mypy on Lib/test/libregrtest	18s
Run mypy on Tools/check-c-api-docs	16s
Run mypy on Tools/peg_generator	16s
Run mypy on Tools/cases_generator	17s
Run mypy on Lib/_pyrepl	19s
Run mypy on Tools/clinic	17s
Run mypy on Lib/tomllib	16s
Run mypy on Tools/build	18s
Run mypy on Tools/jit	17s
	2m 34s

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