mypy

Add a What's New entry for Emscripten (#137035) #9426

Job	Run time
Run mypy on Lib/_pyrepl	18s
Run mypy on Tools/cases_generator	18s
Run mypy on Lib/test/libregrtest	17s
Run mypy on Tools/clinic	19s
Run mypy on Tools/peg_generator	17s
Run mypy on Lib/tomllib	17s
Run mypy on Tools/jit	21s
Run mypy on Tools/build	17s
	2m 24s

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