[bugfix] Fix login error in MW < 1.27
[bugfix] Fix login error in MW < 1.27

My bad, last patch broke their login process. Good we now test
Fandom wiki again and can detect these issues quickly!

Change-Id: I5600dfa32d607cc59f9bd69ebe789d8de6ac3d7e

PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,wikimedia/pywikibot-core