Since cloudpickle 1.4.0 uses ClassVar from this commit, which is only supported by Python 3.5.3 and above.
I'm amateur python developer, so I'm not sure of the best way to address / announce this. But this may be worth fixing because Ubuntu 16.04 ships with python 3.5.1-2, so all users of that version of Ubuntu would be affected as well.
Thanks!