mypy

GH-114610: Fix `pathlib.PurePath.with_stem('')` handling of file exte… #1

Job	Run time
Run mypy on Lib/test/libregrtest	11s
Run mypy on Tools/build/	19s
Run mypy on Tools/cases_generator	16s
Run mypy on Tools/clinic	19s
Run mypy on Tools/jit	22s
Run mypy on Tools/peg_generator	20s
Run mypy on Tools/wasm	10s
	1m 57s

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