This is a simplified process for installing and running certbot-auto on CentOS 6 with Python 3.6.x. I have tried several different approaches to this and this method took the least amount of effort.

yum install -y centos-release-SCL
yum install -y epel-release
yum clean all
yum install -y rh-python36
curl -o /usr/local/bin/certbot-auto http://dl.eff.org/certbot-auto
chmod +x /usr/local/bin/certbot-auto
scl enable rh-python36 bash
certbot-auto help

Results