ci: Add Python 3 to CentOS-6 docker image.
ci: Add Python 3 to CentOS-6 docker image.

In antecipation for migrating to Python 3.

But this time around, don't remove Python 2, so we can continue to build
old branches.

apitrace/apitrace,apitrace/apitrace,apitrace/apitrace,apitrace/apitrace