Update viewer for PyQt5 compatibility

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

skimage/viewer/plugins/base.py

@@ -4,19 +4,11 @@ Base class for Plugins that interact with ImageViewer.
 from warnings import warn
 
 import numpy as np
-
-from ..qt import QtGui, qt_api
-from ..qt.QtCore import Qt, Signal
+from ..qt import QtWidgets, QtCore, Signal
 from ..utils import RequiredAttr, init_qtapp
-from skimage._shared.testing import doctest_skip_parser
-
-if qt_api is not None:
-    has_qt = True
-else:
-    has_qt = False
 
 
-class Plugin(QtGui.QDialog):
+class Plugin(QtWidgets.QDialog):
     """Base class for plugins that interact with an ImageViewer.
 
     A plugin connects an image filter (or another function) to an image viewer.
@@ -101,7 +93,7 @@ class Plugin(QtGui.QDialog):
                  "then the `image_filter` argument is ignored.")
 
         self.setWindowTitle(self.name)
-        self.layout = QtGui.QGridLayout(self)
+        self.layout = QtWidgets.QGridLayout(self)
         self.resize(width, height)
         self.row = 0
 
@@ -124,7 +116,7 @@ class Plugin(QtGui.QDialog):
         the image matches the filtered value specified by attached widgets.
         """
         self.setParent(image_viewer)
-        self.setWindowFlags(Qt.Dialog)
+        self.setWindowFlags(QtCore.Qt.Dialog)
 
         self.image_viewer = image_viewer
         self.image_viewer.plugins.append(self)

skimage/viewer/plugins/color_histogram.py

@@ -1,8 +1,5 @@
 import numpy as np
-try:
-    import matplotlib.pyplot as plt
-except ImportError:
-    pass
+import matplotlib.pyplot as plt
 from skimage import color
 from skimage import exposure
 from .plotplugin import PlotPlugin

skimage/viewer/plugins/plotplugin.py

@@ -1,6 +1,6 @@
 import numpy as np
-from ..qt import QtGui
 
+from ..qt import QtGui
 from ..utils import new_plot
 from .base import Plugin