Almost all major open source Python packages now support both Python 3.x and Python 2.7, and many projects have been supporting these two versions of the language for several years. While we have developed tools and techniques to maintain compatibility efficiently, it is a small but constant friction in the development of a lot of code.
We are keen to use Python 3 to its full potential, and we currently accept the cost of writing cross-compatible code to allow a smooth transition, but we don’t intend to maintain this compatibility indefinitely. Although the transition has not been as quick as we hoped, we do see it taking place, with more and more people using, teaching and recommending Python 3.
The developers of the Python language extended support of Python 2.7 from 2015 to January 1, 2020, recognising that many people were still using Python 2. We believe that the extra 5 years is sufficient to transition off of Python 2, and our projects plan to stop supporting Python 2 when upstream support ends in 2020, if not before. We will then be able to simplify our code and take advantage of the many new features in the current version of the Python language and standard library.
In addition, significantly before 2020, many of our projects will step down Python 2.7 support to only fixing bugs, and require Python 3 for all new feature releases. Some projects have already made this transition. This too parallels support for the language itself, as Python 2.7 releases only include bugfixes and security improvements.
Third parties may offer paid support for our projects on old Python versions for longer than we support them ourselves. We won’t obstruct this, and it is a core principle of free and open source software that this is possible. However, if you enjoy the free, first party support for many projects including the Scientific Python stack, please start planning to move to Python 3.
For all of these reasons, we have pledged to  drop support for Python 2.7 no later than Jan 1st, 2020 , coinciding with the Python development team’s timeline for dropping support for Python 2.7 63.