mypy

`compute-changes.py`: Fix & test `process_changed_files()` #19489

Job	Run time
Run mypy on Tools/cases_generator	19s
Run mypy on Tools/check-c-api-docs	18s
Run mypy on Tools/clinic	18s
Run mypy on Lib/_pyrepl	18s
Run mypy on Lib/test/libregrtest	20s
Run mypy on Tools/peg_generator	17s
Run mypy on Tools/jit	19s
Run mypy on Lib/tomllib	21s
Run mypy on Tools/build	21s
	2m 51s

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