fix pylint warnings

pylint 1.8.3

samples/python/camera_calibration_show_extrinsics.py

@@ -184,7 +184,7 @@ def main():
     extrinsics = fs.getNode('extrinsic_parameters').mat()
 
     import matplotlib.pyplot as plt
-    from mpl_toolkits.mplot3d import Axes3D
+    from mpl_toolkits.mplot3d import Axes3D  # pylint: disable=unused-variable
 
     fig = plt.figure()
     ax = fig.gca(projection='3d')