[cleanup] cleanup scripts/commonscat.py
[cleanup] cleanup scripts/commonscat.py

- use str.format(...) instead of modulo for type specifier
  arguments.
- use single quotes for string literals
- use "+" for concatenating strings in some cases

Change-Id: I303434ee78d2b0c4b04d95eabf43338d78973364

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