Python 3.5 was the target runtime for the Rocky release. The current
target py3 runtime for Stein is Python 3.6, so there is no reason to
keep testing against the older version.

https://governance.openstack.org/tc/reference/runtimes/stein.html#python-runtime-for-stein

Change-Id: I0db6c9cbf2af9ed9c701755a26f721aeac97d4cd