mypy

gh-143930: Tweak the exception message and increase test coverage (GH… #20379

Job	Run time
Run mypy on Lib/test/libregrtest	18s
Run mypy on Tools/cases_generator	16s
Run mypy on Tools/check-c-api-docs	18s
Run mypy on Lib/_pyrepl	17s
Run mypy on Tools/jit	17s
Run mypy on Tools/clinic	17s
Run mypy on Tools/build	20s
Run mypy on Lib/tomllib	18s
Run mypy on Tools/peg_generator	19s
	2m 40s

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