Debugged your code inside lxc-container Ubuntu. Then set up a combat server container with jessie. Added
% cat /etc/apt/sources.list | grep back
deb [arch=amd64] http://mirror.yandex.ru/debian jessie-backports main contrib non-free
Doing an update. Verify via the website that in testing there is a python3 package.5:
Jessie (testing) (python): Interactive high-level object-oriented language (version 3.5)
3.5.3-1: amd64 arm64 armel armhf i386 mips mips64el mipsel ppc64el s390x
But to find it in backports fails.
% apt-t wheezy-backports search python 3.5
Full Text Search... Done
It turns out that the package is in testing but it is not in the backports. How to be in this situation? Need best way to install 3.5 regular means.