[bugfix] import ClosedSite for eval() in cache.py
[bugfix] import ClosedSite for eval() in cache.py

Bug: T225867
Change-Id: I88a7a285633cd3d0181deeafcec6552835490adc

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