site.py: fix intersection of sets of namespaces
site.py: fix intersection of sets of namespaces

Bug: T114563
Change-Id: I6c8ce15dffdabe11a49f51d45b04574d8af94f44

jayvdb/pywikibot-core,smalyshev/pywikibot-core,npdoty/pywikibot,wikimedia/pywikibot-core,Darkdadaah/pywikibot-core,wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,hasteur/g13bot_tools_new,happy5214/pywikibot-core,magul/pywikibot-core,darthbhyrava/pywikibot-local,happy5214/pywikibot-core,Darkdadaah/pywikibot-core,jayvdb/pywikibot-core,magul/pywikibot-core,npdoty/pywikibot,hasteur/g13bot_tools_new,h4ck3rm1k3/pywikibot-core,hasteur/g13bot_tools_new,h4ck3rm1k3/pywikibot-core