[cleanup] cleanup scripts/unusedfiles.py
[cleanup] cleanup scripts/unusedfiles.py

- remove preleading "u" from strings
- use single quotes for string literals
- use str.format(...) instead of modulo for changed files

Change-Id: I41e25cd244526d1b2edc5f3d89c91c512767b5dd

wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core