Re: [PATCH 0/3] Improve CI testing of older systems

> In many cases, we're not testing some of the older Linux distributions
> that we support.  However, we _are_ testing some old Linux distributions
> that we _don't_ support.  That means that it's presently difficult for
> us to make sure our code works on all the systems we want it to and that
> we're not accidentally requiring newer versions of dependencies than we
> claim to support.
> 
> The first patch removes Ubuntu 16.04, which is out of regular LTS
> support.  The second removes the clause in the CI job for that OS.  The
> third adds several new CI jobs to make sure we're adequately supporting
> older Debian and RHEL-compatible distros, and documents the dates at
> which they lose support (so it's easy to know if we need to update
> them).