mypy

Add `perchance`, a soft keyword for exception-tolerant expressions (PoC) #22517

Job	Run time
Run mypy on Lib/tomllib	40s
Run mypy on Tools/check-c-api-docs	36s
Run mypy on Lib/_pyrepl	31s
Run mypy on Lib/test/libregrtest	32s
Run mypy on Tools/clinic	33s
Run mypy on Tools/cases_generator	32s
Run mypy on Tools/build	31s
Run mypy on Tools/peg_generator	31s
Run mypy on Tools/jit	30s
	4m 56s

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