Use yoga stable contraint in tox to release 31.1.0
Use yoga stable contraint in tox to release 31.1.0

We are going to release a new tempest tag 31.1.0 cut an
intermediate release during the Zed development cycle to
make bug fixes available for plugins and other consumers.

Change-Id: Iebd6d6412537959e3e59bf899c3670601733cef3

openstack/tempest,openstack/tempest