Fix build error on Python 2.6
Fix build error on Python 2.6

Build currently fails on 2.6 due to shorthand set notation used
in one place.

Change-Id: If5abc312d88b4567198c5506ffa47b40703a69da

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