wikisource_family.py: Add "pa" to languages_by_size
wikisource_family.py: Add "pa" to languages_by_size

Not having "pa" code in languages_by_size was causing testLinkMethods to
fail on wikisource builds. See
https://travis-ci.org/wikimedia/pywikibot-core/builds/225614750 .

Bug: T163871
Change-Id: Ia2b30a1720a43f7fedc906f0182f098835ede464

hasteur/g13bot_tools_new,jayvdb/pywikibot-core,wikimedia/pywikibot-core,Darkdadaah/pywikibot-core,wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,hasteur/g13bot_tools_new,magul/pywikibot-core,magul/pywikibot-core,happy5214/pywikibot-core,npdoty/pywikibot,happy5214/pywikibot-core,npdoty/pywikibot,hasteur/g13bot_tools_new,Darkdadaah/pywikibot-core,jayvdb/pywikibot-core