mypy

gh-138451: Support custom LLVM installation path #18218

Job	Run time
Run mypy on Lib/tomllib	20s
Run mypy on Tools/build	19s
Run mypy on Lib/test/libregrtest	16s
Run mypy on Tools/cases_generator	17s
Run mypy on Tools/peg_generator	21s
Run mypy on Tools/clinic	19s
Run mypy on Tools/jit	19s
Run mypy on Lib/_pyrepl	20s
	2m 31s

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