mypy

Remove unused :platform: in module's docs (GH-144988) #19587

Job	Run time
Run mypy on Tools/build	18s
Run mypy on Lib/tomllib	17s
Run mypy on Tools/clinic	17s
Run mypy on Tools/cases_generator	18s
Run mypy on Tools/peg_generator	17s
Run mypy on Tools/jit	16s
Run mypy on Lib/_pyrepl	17s
Run mypy on Tools/check-c-api-docs	19s
Run mypy on Lib/test/libregrtest	21s
	2m 40s

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