[DCAE] Set 3.9 for tox run

Address error from
https://jenkins.onap.org/job/dcaegen2-services-ml-prediction-ms-tox-verify-all/11/console

Change-Id: I3545af9c260f52cbc01f9ecbedf5ee6594b159f2
Signed-off-by: vv770d <vv770d@att.com>
Issue-ID: DCAEGEN2-3236
1 file changed