bpo-41110: Fixed lib2to3 reporting some unchanged files as needing modification by AWhetter · Pull Request #21296 · python/cpython
This changes prevents lib2to3 fixers from changing the tree unnecessarily, which results in no visible diff and the file still being reported as needing modifications.
This partially addresses an issue in python-modernize where a non-zero exit code is returned even when no changes to files need to be made: PyCQA/modernize#161
Given that lib2to3 is on its way out, I don't know if this should be fixed in 3.8 and 3.9 only?