Pywikibot is a Python-based framework to write bots for MediaWiki (more information).
scripts/maintenance/diff-checker.py is a file that is executed at every commit test to execute common verification in the edited files/code lines.
Each file in pywikibot repository contain # (C) Pywikibot team, 2008-2017 comment or so. All comments are not updated automatically each 1st January, but only when an edit is made in the code, so it's a way to know when the code was updated.
diff-checker should verify that the edited files contain this up-to-date comment. You can use a regex detection to verify that no outdated comment is still present in the edited file.
You are expected to provide a patch in Wikimedia Gerrit. See https://www.mediawiki.org/wiki/Gerrit/Tutorial for how to set up Git and Gerrit.