mypy

`compute-changes.py`: Fix & test `process_changed_files()` #19596

Job	Run time
Run mypy on Lib/_pyrepl	22s
Run mypy on Tools/peg_generator	24s
Run mypy on Tools/clinic	20s
Run mypy on Tools/cases_generator	22s
Run mypy on Lib/tomllib	22s
Run mypy on Tools/jit	17s
Run mypy on Tools/build	20s
Run mypy on Tools/check-c-api-docs	19s
Run mypy on Lib/test/libregrtest	21s
	3m 7s

Provide feedback