mypy

gh-141510: Document isinstance change for frozendict in What's New (#… #20024

Job	Run time
Run mypy on Tools/clinic	39s
Run mypy on Lib/tomllib	16s
Run mypy on Tools/peg_generator	18s
Run mypy on Tools/cases_generator	20s
Run mypy on Tools/jit	18s
Run mypy on Lib/test/libregrtest	18s
Run mypy on Tools/check-c-api-docs	19s
Run mypy on Lib/_pyrepl	19s
Run mypy on Tools/build	18s
	3m 5s

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