mypy

Update PCbuild/readme.txt to correct the default platform (GH-142337) #18335

Job	Run time
Run mypy on Lib/_pyrepl	21s
Run mypy on Tools/build	19s
Run mypy on Tools/cases_generator	18s
Run mypy on Lib/test/libregrtest	20s
Run mypy on Tools/clinic	18s
Run mypy on Lib/tomllib	17s
Run mypy on Tools/peg_generator	20s
Run mypy on Tools/jit	19s
	2m 32s

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