Support for Python 3

[carlosalberto]

$ pip install sqlalchemy_opentracing
Collecting sqlalchemy_opentracing
  Using cached https://files.pythonhosted.org/packages/6c/b6/b287ef696356ad4007e9a43b6730fe53b8d0a0ed1109c9969f2a87b59497/sqlalchemy_opentracing-0.1.5.tar.gz
Requirement already satisfied: sqlalchemy in ./venv/lib/python3.6/site-packages (from sqlalchemy_opentracing) (1.1.11)
Collecting opentracing<1.2,>=1.1 (from sqlalchemy_opentracing)
  Using cached https://files.pythonhosted.org/packages/40/12/abd02daa323540302016280650123e7d1cc10b448b40b7e40fffc23ce339/opentracing-1.1.0.tar.gz
Collecting futures (from opentracing<1.2,>=1.1->sqlalchemy_opentracing)
  Using cached https://files.pythonhosted.org/packages/4a/f4/418e844d868e34638486732417fb82b05031910059d88b86aaea9c70f699/futures-3.1.2.tar.gz
    Complete output from command python setup.py egg_info:
    This backport is meant only for Python 2.
    It does not work on Python 3, and Python 3 users do not need it as the concurrent.futures package is available in the standard library.
    For projects that work on both Python 2 and 3, the dependency needs to be conditional on the Python version, like so:
    extras_require={':python_version == "2.7"': ['futures']}