Fix lookup of vfuncs in parent classes

https://bugzilla.gnome.org/show_bug.cgi?id=672864

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As subclasses implemented in python override the attribute for the
vfunc, __mro__ has to be used so subclasses of the subclass overriding
methods may find the corresponding VFuncInfo.

Co-Authored-by: Martin Pitt <martinpitt@gnome.org>