Added support for python3 with poll_client

[shu-tom]

I got the following error when using poll_client in Python 3.7.

Traceback (most recent call last):
  File "/Library/Python/3.7/site-packages/libtaxii/scripts/__init__.py", line 428, in __call__
    self.handle_response(r, args)
  File "poll_client.py", line 87, in handle_response
    self.write_cbs_from_poll_response_11(response, dest_dir=args.dest_dir, write_type_=args.write_type)
  File "/Library/Python/3.7/site-packages/libtaxii/scripts/__init__.py", line 359, in write_cbs_from_poll_response_11
    f.write(cb.content)
TypeError: a bytes-like object is required, not 'str'

After changing to encode the content, it worked in Python3 as well.