python3 support claimed but not actually working

[thomasgilgenast]

specifically there are two errors currently breaking python3 compatibility:

the first is the use of urllib when downloading test images, which causes

Traceback (most recent call last):
  ...
  File "...\projectile\server.py", line 306, in main
    get_test_data()
  File "...\projectile\get_test_data.py", line 10, in main
    urllib.urlretrieve(
AttributeError: module 'urllib' has no attribute 'urlretrieve'

the second is the use of StringIO as the buffer for the image data between PIL and the Tornado response, which causes:

Traceback (most recent call last):
  File "...\tornado\web.py", line 1697, in _execute
    result = method(*self.path_args, **self.path_kwargs)
  File "...\projectile\server.py", line 41, in get
    self.get_slices(z, x, y)), s)))
  File "...\projectile\server.py", line 165, in send_image
    image.save(stream, format='png')
  File "...\PIL\Image.py", line 2084, in save
    save_handler(self, fp, filename)
  File "...\PIL\PngImagePlugin.py", line 807, in _save
    fp.write(_MAGIC)
TypeError: string argument expected, got 'bytes'

[thomasgilgenast]

an easy way to avoid issues like this is to have tests for both py2 and py3 managed by something like tox; we currently have zero tests so even a simple smoketest test case would represent a lot of progress here