please note that PyMC3 will drop support for Python 2 in 2019 where we will start targeting Python 3.6 (or later).
The ecosystem around Python 2 is starting to deteriorate (as indicated by various travis issues that keep coming up) and many other libraries (notably, NumPy and Pandas) also announced ending support for Python 2 in 2019.
If you haven’t yet, you should upgrade to Python 3 soon, it is better in almost every aspect.
Thanks for your support,
The PyMC developers