updated to work for python3

[Salil999]

Used 2to3 -w for client.py and example.py. Updated the unit_test.py library to account for some Python3 imports.

However, I'm not 100% sure about the relative paths for importing the JSON files in unit_test.py. I had to add

import sys
sys.path.append(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
from client import DiffbotClient

just before importing the client to resolve some directory/relative import issues. However, all the unit tests passed once those were resolved. I removed the code that appends to PATH before committing as well.

My text editor also follows the Pep8 standard for Python3 formatting, so I hope that helps as well 👍

[Salil999]

Closing as this is stale