[cleanup] cleanup scripts/clean_sandbox.py
[cleanup] cleanup scripts/clean_sandbox.py

- use str.format(...) instead of modulo for type specifier
  arguments.
- use single quotes for string literals
- remove preleading "u" fron strings

Change-Id: Id754b57036f93d719d8e1e246dff51816a95881b

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