Adding 3.5 to the PYTHON_TARGETS="python3_5" should be enough.
Why is this needed?
Some packages try to import it even if running under py3.5 already, but you are right, my concrete case of a failing urllib3 was actually caused by ImportError: /usr/lib64/python3.5/lib-dynload/_ssl.cpython-35m-x86_64-linux-gnu.so: undefined symbol: SSLv2_method So I'm not really sure if the support is required, but I can imagine some packages blindly import backports and don't check if it's no longer required. Still, this should be fixed in the package then.