[Python 3] Use print() as function instead of statement

Summary:
print() has been a function since Python 2.6; the old behaviour was
dropped in Python 3.0.

To use the named file argument in Python 2, we need to import
print_function from the future module.

Reviewers: machniak, mollekopf

Reviewed By: mollekopf

Differential Revision: https://git.kolab.org/D3229