
In Train, we will use python 3.6 and 3.7 for python3 runtime in our gate jobs [1]. This commit drops python 3.5 and adds python 3.7. [1] https://governance.openstack.org/tc/reference/runtimes/train.html Change-Id: I9628ad9293ea8ccae7d39af564b6713e60d0d4b5