brain_gi: magic methods are not recognized for Gtk objects

[lovetox]

Steps to reproduce

from gi.repository import Gtk

model = Gtk.ListStore(str)
model.append(["asd"])
a = model[0][0]
print(a)

See also https://gitlab.gnome.org/GNOME/pygobject/issues/361

Current behavior

This yields E1136: Value 'model' is unsubscriptable (unsubscriptable-object)

This is a false positive, the ListStore Widget implements __setitem__()

Version

pylint 2.4.4
astroid 2.3.3
Python 3.5.4 (v3.5.4:3f56838, Aug 8 2017, 02:17:05) [MSC v.1900 64 bit (AMD64)]