Dropping support for py35

As per below ML thread, we are good to drop the py35
support now:
http://lists.openstack.org/pipermail/openstack-discuss/2019-April/005097.html

[1]http://lists.openstack.org/pipermail/openstack-discuss/2019-April/004647.html
[2]
https://governance.openstack.org/tc/reference/runtimes/stein.html
https://governance.openstack.org/tc/reference/runtimes/train.html

Change-Id: Icf0980b8f1a8d884b05621f5298c13cad5f05387
1 file changed