Gevent looks like a nice system, but as far as I can tell doesn't support Python 3. Does anyone have a sense of when support is planned?

Unfortunately, quite a few critical libraries still only Python 2.7. Many of them, including gevent, are definitely trying to port to Python 3 but it's sometimes not an easy task.

It's definitely harder than your average library, since greenlet (the library gevent is based on) involves such direct manipulation of the call stack / other CPython internals. I'd love to see it, but I don't think it's coming any time soon :(

After a short googling I found this saying there's a python 3 fork https://github.com/surfly/gevent/issues/38

No idea if it's getting merged soon or at all

I expect there will be some serious push for that. Not sure from which side, but at least the web frameworks and openstack will be interested. (unless they'll just migrate to tulip instead)