[cleanup] cleanup scripts/category.py
[cleanup] cleanup scripts/category.py

- use str.format(...) instead of modulo for type specifier
  arguments.
- use "+" to concatenate strings in some cases
- remove preleading "u" from strings

Change-Id: I34a808cd60a7e966991bbfbde3f2a67f55cbff67

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