Decode JSON content when parsing API results

[tomwijnroks]

On systems with Python 3.5.3 (Debian 9) the Parser fails to decode the content:

  File "/usr/local/lib/python3.5/dist-packages/exonetapi/result/Parser.py", line 17, in __init__
    self.__json = json.loads(self.__data)
  File "/usr/lib/python3.5/json/__init__.py", line 312, in loads
    s.__class__.__name__))
TypeError: the JSON object must be str, not 'bytes'

Adding .decode() fixes this. I tested this change with Python 3.5.3 and Python 3.7.3.

It has been fixed since Python 3.6