Use print as a function for Python 3 [PEP 3105]

Use print as a function for Python 3 [PEP 3105]

Import print_function from the __future__ module
to activate this for Python 2.  This works as far back as Python 2.6.0a2:
   https://docs.python.org/2/library/__future__.html

[PEP 3105] https://www.python.org/dev/peps/pep-3105/