[FIX] checkimages: Expect Timestamp instance
[FIX] checkimages: Expect Timestamp instance

The checkimages script uses since d298facc the FileInfo class returned by
FilePage instances. And these classes always return Timestamp instances so the
script needs to be adjusted to that.

Bug: T105188
Change-Id: I2b6257af05a2ecc620d718872a0b6d3f0a7f64e4

Darkdadaah/pywikibot-core,icyflame/batman,magul/pywikibot-core,jayvdb/pywikibot-core,happy5214/pywikibot-core,emijrp/pywikibot-core,wikimedia/pywikibot-core,happy5214/pywikibot-core,hasteur/g13bot_tools_new,npdoty/pywikibot,Darkdadaah/pywikibot-core,darthbhyrava/pywikibot-local,smalyshev/pywikibot-core,hasteur/g13bot_tools_new,valhallasw/pywikibot-core,h4ck3rm1k3/pywikibot-core,npdoty/pywikibot,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,VcamX/pywikibot-core,trishnaguha/pywikibot-core,jayvdb/pywikibot-core,h4ck3rm1k3/pywikibot-core,TridevGuha/pywikibot-core,magul/pywikibot-core,hasteur/g13bot_tools_new