If I was going to start an open source project using Python what version should I use to ensure that the vast majority of users can use it on their system?

I'm the kind of person who quickly jumps to the next version (which I'll do when Python 3 comes out) but many people may be more conservative if their current version seems to be working fine. What version would hit the sweet spot but still allow me to enjoy the newest and coolest language enhancements?

python compatibility
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