.eslintrc (if they exist) files should automatically be renamed to .eslintrc.json.
See https://gerrit.wikimedia.org/r/c/labs/libraryupgrader/+/523340 for an example patch that implemented a different kind of fixer. You likely want to use the shutil.move function to implement this.
Affects: https://codesearch.wmflabs.org/search/?q=%5BA-z%5D&i=nope&files=.eslintrc%24&repos=