I can think of one reason for using Python 2 -- if you are creating a project that you want to share with others, there are a lot of enterprise systems that are still on Python 2.x. I've had the experience a number of times of finding a program that seems to do exactly what I want, but I can't deploy it on my production servers because they are all running RHEL 6 with Python 2.6.
Of course, it is possible to install multiple versions side-by-side, and edit the scripts so they all start with #!/usr/local/bin/python3 or something like that, but it is just another barrier of entry. Not to mention that then there are most likely a bunch of additional modules that have to be tracked down to get something to work.
(Actually, come to think of it, this really isn't a python specific issue, this is really a general plea that if you are making something that is geared toward sysadmins, make sure that there is an easy enough installation method available for at least the currently supported versions of RHEL/Centos, Ubuntu LTS, and Suse Enterprise).
The implication that you cannot use Python 3 "if you are creating a project that you want to share with others" is false. Python 3 code is routinely shared and used.
There are a lot of enterprise systems that are still on COBOL, but that wouldn't make it any less ridiculous to say that you can't use Python if you are creating a project that you want to share with others.
In addition to what others said, if you write code for python 3 making it compatible with python2.7 is relatively painless task (from my own experience) due to Python 2.7 having py3 functionality back ported.
Regarding python 2.6 this was discontinued back in 2013, as far as I know all redhat versions that used 2.6 are no longer supported unless you're paying for extended support. So I'm not convinced it is a good excuse.
Also if you use Java for example and you want to use JDK7 or JDK8 it doesn't matter that RH distro uses old version you'll find a proper RPM (or build one) to use it.
RHEL 6 uses python 2.6, and is supported till 2020 (not counting the extended support). Of course, if an app only needs a single host or two, its no problem to have an updated OS for that app. But certain app types, such as backup and monitoring apps, don't do any good unless they run on the majority of systems that a site has. In my case, I couldn't even try out several popular backup systems, as they either required Python 3, or had other library requirements that weren't available on the older enterprise systems.
Of course, it is possible to install multiple versions side-by-side, and edit the scripts so they all start with #!/usr/local/bin/python3 or something like that, but it is just another barrier of entry. Not to mention that then there are most likely a bunch of additional modules that have to be tracked down to get something to work.
(Actually, come to think of it, this really isn't a python specific issue, this is really a general plea that if you are making something that is geared toward sysadmins, make sure that there is an easy enough installation method available for at least the currently supported versions of RHEL/Centos, Ubuntu LTS, and Suse Enterprise).