mypy

gh-151112: Move an `assert` that may fail in `cfg_builder_check` (#15… #21839

Job	Run time
Run mypy on Lib/_pyrepl	20s
Run mypy on Tools/check-c-api-docs	22s
Run mypy on Tools/cases_generator	21s
Run mypy on Tools/peg_generator	19s
Run mypy on Tools/clinic	18s
Run mypy on Tools/jit	23s
Run mypy on Lib/tomllib	18s
Run mypy on Tools/build	21s
Run mypy on Lib/test/libregrtest	19s
	3m 1s

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