Add support for KDE neon

[lleaff]

When running apt-select on KDE neon you get a "Must be an Ubuntu OS" error.
KDE neon is based on Ubuntu LTS but there doesn't seem to be a standard way to tell from the CLI.

$ cat /etc/issue
KDE neon 5.9 \n \l

$ lsb_release -a
Distributor ID: neon
Description:    KDE neon User Edition 5.9
Release:        16.04
Codename:       xenial

There is a hint in the kernel version name but I have doubts on how reliable that info is:

$ uname -v
#88-Ubuntu SMP Wed Mar 8 16:34:45 UTC 2017

Thus I added a check against 'neon' in the code.

[jblakeman]

Thanks for submitting this.

I'm hesitant to keep a static list of Ubuntu variants that follow this behavior. Looking at the output of uname -v on 12.04, 14.04 and 16.04, the first fields all seem to match the pattern: #[0-9]+-Ubuntu, so one might think it would be at least somewhat reliable to parse on most if not all Ubuntu variations.

I lean toward first parsing the output of uname -v as a fall back (if dist != 'Ubuntu') before raising the ValueError.

[lleaff]

I can understand these reservations.
With the kernel version-check, I'd be afraid of false positives, e.g. a distro that somehow uses a kernel compiled for Ubuntu but is actually not Ubuntu-like at all in the userspace.
Either way, what do you think of giving the user the option to bypass this Ubuntu check? Could be with a confirmation prompt, or a "--force" style switch.

[jblakeman]

I like the idea of --force. However, we'd need to force the user to provide a default series (e.g. Xenial) if they aren't using --ping-only. #39 mentions this.