gh-126105: Fix crash in `ast` module, when `._fields` is deleted (#12… #184

Job	Run time
Run mypy on Lib/_pyrepl	24s
Run mypy on Lib/test/libregrtest	24s
Run mypy on Tools/build	24s
Run mypy on Tools/cases_generator	19s
Run mypy on Tools/clinic	24s
Run mypy on Tools/jit	22s
Run mypy on Tools/peg_generator	21s
Run mypy on Tools/wasm	23s
	3m 1s

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