message: relocate import av in pptsn_reader
diff: import numpy as np
  -import av
  import cv2
  import pickle
  -import decord as de
  import math
  import random
  import os
  from PIL import Image
  -import SimpleITK as sitk
  -
  try:
  import cPickle as pickle
  from cStringIO import StringIO
  + import av
  + import decord as de
  + import SimpleITK as sitk
  except ImportError:
  import pickle
  from io import BytesIO
  