[Tests] Fix for isort config package handling

* Force gtk modules to be third_party for tox/travis testing.

tox.ini

@@ -96,7 +96,9 @@ whitelist_externals =
 commands =
     isort --version
     python -c "import subprocess, sys; output = subprocess.check_output(\
-        'isort -q --diff --recursive deluge docs packaging *.py', shell=True);\
+        'isort -q --diff --recursive deluge docs packaging *.py\
+        -o pygtk -o gtk -o gobject -o gtk.gdk -o pango -o cairo -o pangocairo\
+        ', shell=True);\
         print output; sys.exit(bool(output))"
 
 [testenv:flake8]