Add py3k compatibility for core widget and overlayplugin.

Dictionary keys are treated differently in py3k, causing the Canny
plugin to not work on py3k.

skimage/viewer/plugins/overlayplugin.py

@@ -45,7 +45,7 @@ class OverlayPlugin(Plugin):
         self._overlay_plot = None
         self._overlay = None
         self.cmap = None
-        self.color_names = self.colors.keys()
+        self.color_names = [c for c in self.colors.keys()]
 
     def attach(self, image_viewer):
         super(OverlayPlugin, self).attach(image_viewer)

skimage/viewer/widgets/core.py

@@ -232,7 +232,7 @@ class ComboBox(BaseWidget):
         self.name_label.setAlignment(QtCore.Qt.AlignLeft)
 
         self._combo_box = QtGui.QComboBox()
-        self._combo_box.addItems(items)
+        self._combo_box.addItems([i for i in items])
 
         self.layout = QtGui.QHBoxLayout(self)
         self.layout.addWidget(self.name_label)