Gevent said it looks like they're supporting Python3 as of the 1.1 release - http://www.gevent.org/whatsnew_1_1.html
I've used requests with Python3 quite a bit.. I have no idea why it's not listed as supported on the WoS, but their page shows it as working since 2012. https://pypi.python.org/pypi/requests/
Protobugs looks like it's now Py3 compatible, per the devs- https://github.com/google/protobuf/issues/646
ThriftPy has supported Python3 for a while (although ThriftPy is slower than the official lib). Apache Thrift has recently begun working with Python3 as well, however (https://issues.apache.org/jira/browse/THRIFT-1857)
I run Ansible/etc in their own virtualenvs, so they aren't part of system python for me, but you're right - I did have to write an Ansible module recently, and I recall that I did have to use Py2.
The only major package I recall having a problem with was PIL - Eventually I moved to Pillow as a drop-in replacement.
[1]: http://docs.python-requests.org/en/latest/
[2]: http://docs.python-requests.org/en/latest/#feature-support
If you're willing to do a bit more than "pip install x", well I guess it doesn't work, back to py2, you can use almost everything on py3. (and yes requests is ported too)
Moreover, in machine learning Python 2.7 is still considered the default version of Python. (E.g.: some part of OpenCV need Python 2.7; until recently Spark supported only Python 2.7.)
The list I've gone off of is https://python3wos.appspot.com/. I use python a lot, and only now in late 2015 I might finally use python3 if starting a new project. When I started a new project last year, we used python2.
The biggest hold-out for me was gevent, which was only released 5 months ago. Gunicorn with gevent workers is my preferred stack for running python apps.
If you use protobufs or thrift, those both aren't yet on python3.
The wall of shame currently lists requests as not working on python3, though I think that might be a fluke.
These ones might not be a deal-breaker since you can have a separate environment for infrastructure & app code, but for some reason a lot of the infrastructure tools still haven't updated to python 3 (supervisor, ansible, fabric, graphite).
All together, it adds up to a not-insignificant number of things that aren't yet on python3. And even if nothing you use when you first start a project is python2 only, you have no idea what libraries you might need or want in the future and if those might be python2 only.