Our standards are still evolving in some sense, however, so the rule is typically:

If you're touching a file, you should reformat the things you're changing. If you find a file that needs wholesale changes (very rare: we all install PEP8 linters in our editors), make them in a separate commit.

As an example of evolution, we now use an import style that puts multiple imports from the same module on separate lines. Adding a new import only changes one line, and reordering the imports if someone hasn't done so doesn't really confuse things.