I'm a Python developer and I still only write in Python 2. I'll sometimes release projects and make them compatible with both Python 2 and 3, but for any personal or professional (non-open source) work I use Python 2.

Python 2 still has too much pull. Lots of smaller open source projects and third party libraries use 2, as do little snippets and examples all around the Internet. I've never found a case where a project I wanted to use, modify, or borrow from supported 3 but not 2, but the inverse is unfortunately still quite common.

I think Python 3 is a better language over all, but it's easier for me as a developer to keep using what I know will be supported and compatible.