Fix tempest-slow-py3: use the correct inheritance chain

Derive the job from the py3 version of the multinode base job,
otherwise several definitions are lost.
The more visible symption of this issue is the wrong usage of
USE_PYTHON3=False when the job is used on stable/ussuri.

Closes-Bug: #1935956
Change-Id: I603f952f18851828ca681ddcc82c54c4d2fe66e6
1 file changed