update cliff to 3.5.0 support python3.8
update cliff to 3.5.0 support python3.8

[1] https://pypi.org/project/cliff/3.5.0/

Change-Id: I1d6fb5ca4e0856a2766a69f7fce9b56d7253f754

openstack/python-troveclient,openstack/python-troveclient