We want Python 3.5+ to be available everywhere so various processes can start using it. The debian-base Dockerfile is shared by Debian 7 and 9 images. Debian 9 ships with Python 3.5 and after the previous commit, we have a Python 3.5 package for Debian 7. So we simply install the "python3.5" package to get Python on all the Debian images. MozReview-Commit-ID: 9ZmoSxtHWTZ