So let us start by setting the stage. Why does it matter? Isn't running the latest version with the most support a normal behavior? Well, not in the Python ecosystem. Python 2 had some problems. And they were the kind of problems that could not be addressed in a way that was backwards compatible. So the decision was made to put backwards compatibility breaking changes into Python 3. Unfortunately this was coupled with some early Python 3 releases that suffered in terms of performance. This created a schism in the Python Community. Many people choose to stick with Python 2. But enough of the past, here is the current situation: Python 2.7 is in it's last days and Python 3 is, in every way, better than Python 2. As PEP 373 tells us: "There will be no Python 2.8". As of when this is published, Python 2.7 will not be maintained past 2020. That means by the time you read this, Python 2.7 is either on it's way out, or already unsupported. Even if it is only on it's way out, the primary focus of development from the core developers and the rest of the community is on Python 3. Most, if not all, of the actively developed development tools for Python support version 3. And the same goes for packages. All of the top packages have been ported to Python 3, or have been replaced by ones that support it.
Not only do I attempt to focus on Python 3, I highly encourage you to try and keep up with the latest version of Python if at all possible. Not only is it important to do so for security related fixes, but you also will realize many advantages in features and functionality:
For more information you can give my DerbyCon 2016 Talk: Python 3, It's Time presentation a watch.
Good Luck & Happy Hacking,