Drop Python2 support.

samples/python/plane_tracker.py

@@ -21,14 +21,6 @@ Keys:
 Select a textured planar object to track by drawing a box with a mouse.
 '''
 
-# Python 2/3 compatibility
-from __future__ import print_function
-import sys
-PY3 = sys.version_info[0] == 3
-
-if PY3:
-    xrange = range
-
 import numpy as np
 import cv2 as cv
 
@@ -104,7 +96,7 @@ class PlaneTracker:
         matches = [m[0] for m in matches if len(m) == 2 and m[0].distance < m[1].distance * 0.75]
         if len(matches) < MIN_MATCH_COUNT:
             return []
-        matches_by_id = [[] for _ in xrange(len(self.targets))]
+        matches_by_id = [[] for _ in range(len(self.targets))]
         for m in matches:
             matches_by_id[m.imgIdx].append(m)
         tracked = []