Have you ever wondered how difficult it might be to switch Sage from Python 2.7 to Python 3.x? Some students in my Sage course made a webpage that summarizes the Python 3 support status of the Python packages on which Sage depends.
Matplotlib doesn't support Python 3.x at all yet.
Mercurial doesn't support Python 3.x, and they don't have any plans to do so. (Which is another point in favor switching Sage to GIT.)