python-simplestreams is pinned to a commit from 2018

[j5awry]

there are a few bugs listed on simplestreams usage. in digging more into pycloudlib's sstream requirements, i found that the pinned commit is from 2018

"python-simplestreams @ git+https://git.launchpad.net/simplestreams@21c5bba2a5413c51e6b9131fc450e96f6b46090d", # noqa

commit 21c5bba2a5413c51e6b9131fc450e96f6b46090d
Author: Daniel Watkins <daniel.watkins@canonical.com>
Date:   Thu Dec 6 15:31:01 2018 +0000

    Add 'ubuntu' alias to CURRENT_LTS.
    
    This indicates the best "ubuntu" option available to users if they have
    no other preference

1. Looking at usage, there are 2 direct uses of streams now, lxd and ec2
2. #266 is opened to remove the ec2 case, as SSM parameter store is how Canonical is advertising publicly for finding images.
3. I'm unsure the lxd case. it looks like it's to find an image before launching, and then launching with a specific hash, instead of using lxd built-in aliases (ubuntu:$SUITE for release and ubuntu-daily:$SUITE for dailies).
4. the query is hardcoded to the same URLs that lxd is already using, without an override (which would be helpful for lxd minimal, so providing a new stream via lxc remote add)

First there should be investigation if it is possible to take upstream simplestreams default branch and tip rather than pin. Then there should be some discussion on the lxd cases. It's possible that simplestreams may not be a required package, but rather something that can be reference if someone wants to use something other than an alias. or removing simplestreams and creating your own query, as it's "just json", thus removing an unpackaged upstream dependency