RHEL/CentOS 6 instructions should mention enabling SCL repository for new Python

Recently certbot changed its requirements on Python and started requiring the Software Collections (SCL) repository in order to run, see https://github.com/certbot/certbot/pull/7519.

Whilst there's some form of a migration path (the certbot script could be more explicit about how one installs the SCL repository), new users will be left with a non-working install until they also figure it out. Here's what I did on RHEL6 to enable the repo:

```sh
subscription-manager repos --enable=rhel-server-rhscl-6-rpms
```

I don't have a CentOS 6 machine to try/test corresponding install instructions on, otherwise I would create a PR. There's docs at https://www.softwarecollections.org/en/scls/rhscl/rh-python36/ but they appear to be RHEL/CentOS 7 specific.