A new version of flake8-print (3.1.0) has been released about 4 hours ago which might have caused this issue.
Change 409704 had a related patch set uploaded (by Dalba; owner: Dalba):
[pywikibot/core@master] tox.ini: Ignore newly detected flake8-print errors
Change 409704 merged by jenkins-bot:
[pywikibot/core@master] tox.ini: Ignore or fix newly detected flake8-print errors