Use None instead of 'none' for qt backend

skimage/viewer/qt/__init__.py

@@ -12,7 +12,7 @@ if qt_api is None:
             import PyQt4
             qt_api = 'pyqt'
         except ImportError:
-            qt_api = 'none'
+            qt_api = None
             # Note that we don't want to raise an error because that would
             # cause the TravisCI build to fail.
             warnings.warn("Could not import PyQt4: ImageViewer not available!")