mypy

gh-127750: Fix annotations in singledispatchmethod signature tests (G… #19017

Job	Run time
Run mypy on Tools/build	18s
Run mypy on Lib/_pyrepl	19s
Run mypy on Tools/clinic	18s
Run mypy on Lib/tomllib	19s
Run mypy on Tools/check-c-api-docs	17s
Run mypy on Tools/jit	20s
Run mypy on Tools/cases_generator	20s
Run mypy on Lib/test/libregrtest	18s
Run mypy on Tools/peg_generator	17s
	2m 46s

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