Bug 1449629 - Install Python 3.5 in debian-base; r=glandium

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