[bugfix] fix syntax error vor py 2.6
[bugfix] fix syntax error vor py 2.6

{'alpha', '-alpha'} is a set construction for python > 2.6 but
breaks with invalid syntax for previous version. Use tuple instead.

https://travis-ci.org/wikimedia/pywikibot-core/jobs/112297292

Change-Id: I6abfe8498eb2603d344e29bfae64e567319b5669

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