Comment 4 for bug 1835036

Got the same issue on Ubuntu 18.04 with OpenStack Train client:

$ openstack
WARNING: Failed to import plugin clustering.
(openstack)

Installed python3-openstackclient and python3-zunclient via the Ubuntu Cloud Archive PPA.

$ pip3 list | grep openstackclient
python-openstackclient (4.0.0)

$ pip3 list | grep zunclient
python-zunclient (3.5.0)

Manual import of zunclient library in python3 is possible:

$ python3
Python 3.6.8 (default, Oct 7 2019, 12:59:55)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import zunclient
>>>