Update viewer for PyQt5 compatibility

Conflicts:
	skimage/viewer/tests/test_plugins.py
	skimage/viewer/tests/test_tools.py

skimage/viewer/utils/core.py

@@ -1,28 +1,17 @@
 import warnings
 
 import numpy as np
+from skimage.viewer.qt import QtGui
+import matplotlib as mpl
+from matplotlib.figure import Figure
+from matplotlib import _pylab_helpers
+from matplotlib.colors import LinearSegmentedColormap
 
-from ..qt import qt_api
-
-try:
-    import matplotlib as mpl
-    from matplotlib.figure import Figure
-    from matplotlib import _pylab_helpers
-    from matplotlib.colors import LinearSegmentedColormap
-    if not qt_api is None:
-        from matplotlib.backends.backend_qt4 import FigureManagerQT
-        from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg
-        if 'agg' not in mpl.get_backend().lower():
-            print("Recommended matplotlib backend is `Agg` for full "
-                  "skimage.viewer functionality.")
-    else:
-        FigureCanvasQTAgg = object
-        LinearSegmentedColormap = object
-except ImportError:
-    FigureCanvasQTAgg = object  # hack to prevent nosetest and autodoc errors
-    LinearSegmentedColormap = object
-
-from ..qt import QtGui
+from matplotlib.backends.backend_qt4 import FigureManagerQT
+from matplotlib.backends.backend_qt4agg import FigureCanvasQTAgg
+if 'agg' not in mpl.get_backend().lower():
+    warnings.warn("Recommended matplotlib backend is `Agg` for full "
+          "skimage.viewer functionality.")
 
 
 __all__ = ['init_qtapp', 'start_qtapp', 'RequiredAttr', 'figimage',

skimage/viewer/utils/dialogs.py

@@ -1,6 +1,6 @@
 import os
 
-from ..qt import QtGui
+from skimage.viewer.qt import QtGui
 
 
 __all__ = ['open_file_dialog', 'save_file_dialog']