mypy

Mention _Float16 (type from Annex H of the C23) in the struct docs (#… #20224

Job	Run time
Run mypy on Lib/_pyrepl	16s
Run mypy on Tools/cases_generator	16s
Run mypy on Tools/peg_generator	19s
Run mypy on Tools/clinic	16s
Run mypy on Lib/tomllib	18s
Run mypy on Tools/build	21s
Run mypy on Tools/jit	17s
Run mypy on Tools/check-c-api-docs	16s
Run mypy on Lib/test/libregrtest	18s
	2m 37s

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