Updating python to the Debian's testing branch seems to resolve the issue
The 'issue' is a harmless warning not justifying to break Debian's package management!
For anyone else reading this: please simply do this instead
Code
wget -O /usr/lib/python3.5/weakref.py https://raw.githubusercontent.com/python/cpython/9cd7e17640a49635d1c1f8c2989578a8fc2c1de6/Lib/weakref.py
The harmless warning will disappear too but you don't need to fiddle around with adding Debian testing to your apt sources and especially do NOT need to fiddle around with apt pinning which is dangerous especially on the RPi where we already had to pin in a rather weird way to get a marriage between RPi kernel/bootloader stuff and Debian's own package sources.