mypy

gh-145410: Add _sysconfig.get_platform() function (#146145) #20025

Job	Run time
Run mypy on Tools/peg_generator	18s
Run mypy on Lib/tomllib	17s
Run mypy on Tools/jit	59s
Run mypy on Lib/_pyrepl	18s
Run mypy on Tools/check-c-api-docs	16s
Run mypy on Tools/clinic	16s
Run mypy on Tools/build	18s
Run mypy on Tools/cases_generator	57s
Run mypy on Lib/test/libregrtest	19s
	3m 58s

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