mypy

gh-149663: fix typo in `unittest` docs (#149670) #21268

Job	Run time
Run mypy on Lib/test/libregrtest	45s
Run mypy on Tools/build	39s
Run mypy on Tools/clinic	44s
Run mypy on Lib/_pyrepl	44s
Run mypy on Tools/peg_generator	47s
Run mypy on Tools/jit	49s
Run mypy on Lib/tomllib	44s
Run mypy on Tools/check-c-api-docs	42s
Run mypy on Tools/cases_generator	44s
	6m 38s

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