Unify print() usage (#684) - #649

First of all, I think it would be better to have one PR to fix all print statements.

Secondly, the change in this PR doesn't work correctly without from __future__ import print_function. In Python 2 using print() will print () without the aforementioned import.

Ah, so true. Then I will add the import

Because print statement is used mainly on the development tools, that would put the future installation requirement to the requirements-dev.txt. But if need to use the future in places which are used by the library itself then the future installation requirement should go to requirements.txt. I do not see that far, that I could say for sure for which one it should be put. Because not the need is only on the dev tools, should we put the installation requirement only in the requirements-dev.txt?

from __future__ import print_function works out-of-the-box since Python 2.6. In fact, all __future__ imports are handled by Python itself.

Closed on favor of #684