python3 fixes for download_train.py

[rbarman]

A few fixes are required to download_train.py if anyone is using python3.

Current dependency versions (using google colab):

• python : 3.6.7
• opencv-python : 3.4.5.20

Fixes:

1. Use urllib.request instead urllib
   - see https://stackoverflow.com/questions/3969726/attributeerror-module-object-has-no-attribute-urlopen
2. Open the csvs with 'r' instead of 'rb'
   - see https://stackoverflow.com/a/28974410/2475380
3. cv2 needs integers when resizing
   - I get TypeError: integer argument expected, got float when trying to resize images in download_and_resize()
   - One solution is to use integer division like: height = (640 * img.shape[0]) // img.shape[1]

I have these changes ready. However I am not sure I should send in a pull request if the main users in this repo use python2.7 and these changes would cause them issues.