Wouldn't call 28% "largely ignored", rather "still lagging behind". Thing is, 3.6 looks like the real deal breaker here, what Python 3.0 should've been. With asyncio, compact dicts and scandir I've gotten to the point where I can almost justify to my employer why, if everything is still working, we should move to Python 3.6 ("all will be faster" -- I know I'm kind oversimplifying here, but my boss pretty much doesn't understand software and just wants results/money).
There's also the sentiment of "I don't know if [insert module here] will be supported", which has become mostly baseless fear, but people still think Py3 support is lacking (when it's not![0]).
Edit: when I posted this comment, the link was titled "Python 3 largely ignored" (not the article per se, but the submitted link had been titled that way). It has been changed, but this was a bit of important context for my comment.
Not sure why supervisor matters, it's just a program. It's the same thing as Homebrew running Ruby - totally doesn't matter what version they're using.
Of course, specific projects might not be covered, but I've seen a lot of people go "ok, just to be safe I'll use 2.7 in case down the road I get to need anything that's Py2 specific".
In your case you know exactly what your needs are, and that they're unsupported in Python 3, but I'm sure that many of those 70% Py2 projects could run just as well in Py3.
Can't speak for gp, but we're using supervisor inside Docker containers as lightweight init process. In our case the main advantage of Docker containers is ease of installation and consistent environment - so we don't always use just one process per container.
Systemd on the other hand is just something that is being forced down our throats. :-/
Some research on most of the projects which don't:
supervisor: Application supervisor, not a widely used library - it's probably in the top 360 since people install it using pip instead of the distro's package manager
carbon, graphite-web and diamond: Components of the time series DB Graphite, see supervisor
Thrift: Apparently there's a Python 3 pull request? [1]
ansible: Difficult story since they have to support old systems which only support Python 2, so no point in migrating to Python 2 until those are no longer in use (that includes Python 2.4!).
This is a really unique use case for Python so it's hard to compare it to the rest. The part of Ansible which runs on the controller has been ported already [2].
google-apputils: There are Debian packages for Python 3, so probably missing the PyPi metadata [3]. Appears to be unmaintained.
aspen: Niche web framework. They seem to be working on Python 3 support [4].
newrelic_plugin_agent: see supervisord
python-cjson: Unmaintained drop-in JSON library that can be replaced with something else with no effort [5].
python-openid: There's a Python 3 fork which looks nicer than the original [6].
m2crypto: Porting effort underway, but mostly an issue for existing cde - there's no reason to use it for new projects since python-cryptography is better in all regards
python-ldap: Issue for many enterprise Django projects (this is the one library in the list that I personally use), but there's a Python 3 fork so it's fine [7].
INITools: Abandoned, it used to be pip dependency so that's probably why it's in this list (?).
marionette: see supervisord
pathtools: moved to stdlib in Python 3.
So, nothing major left in the top 360.
All of the huge blockers like Twisted and Paramiko are now green and I migrated all my stuff over.
The Fedora porting project has some great insight into how it's going in the real world:
In general, no major libraries left, but lots of grunt work that needs to be done.
Some stuff not on the list, that I encountered:
python-dajax: Turns out it's a bad idea to put frontend logic in your backend code, but that doesn't make the dependency go away (great opportunity to finally undo that mistake in my code)
wxPython: They decided to do a full rewrite instead of a port so it takes some time. Qt, I guess?
There's also the sentiment of "I don't know if [insert module here] will be supported", which has become mostly baseless fear, but people still think Py3 support is lacking (when it's not![0]).
[0] https://python3wos.appspot.com/ -- also, take notice how 9 of this guys come from here [1]
[1] http://mozbase.readthedocs.io/en/latest/
Edit: when I posted this comment, the link was titled "Python 3 largely ignored" (not the article per se, but the submitted link had been titled that way). It has been changed, but this was a bit of important context for my comment.