mypy

`compute-changes.py`: Fix & test `process_changed_files()` #19473

Job	Run time
Run mypy on Lib/_pyrepl	18s
Run mypy on Tools/cases_generator	16s
Run mypy on Tools/check-c-api-docs	22s
Run mypy on Lib/tomllib	19s
Run mypy on Tools/clinic	18s
Run mypy on Tools/peg_generator	16s
Run mypy on Lib/test/libregrtest	22s
Run mypy on Tools/jit	20s
Run mypy on Tools/build	25s
	2m 56s

Provide feedback