Add Python3 xena unit tests
Add Python3 xena unit tests

This is an automatically generated patch to ensure unit testing
is in place for all the of the tested runtimes for xena.

See also the PTI in governance [1].

[1]: https://governance.openstack.org/tc/reference/project-testing-interface.html

Change-Id: Ifb90a8ac5c35f6f6bfa7df9ccb6e1816fad59dde

openstack/oslo.serialization