[FIX] ItemPage: Avoid changing dict in toJSON
[FIX] ItemPage: Avoid changing dict in toJSON

In toJSON it changed the claims dict while iterating over it. As with
Python 3 the iterator is just a view to the current state and thus this
is not allowed.

Change-Id: Ib6dd67f8afdc90c576b2d433b24059f339ae84fa

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