I'm really not a big expert on pywikibot, but from what I know, its users usually update from Gerrit and rely more on the Gerrit version number based on the last commit. If you got pywikibot from Gerrit, you can check it by running
git log -1.
As far as I can see, the usage of the message "cosmetic_changes-append" has changed recently, and I think that if you update to the latest version from Gerrit, the problem will be fixed.
If you try this and it's still not fixed, then I'll try to involve pywikibot developers.