That fear held me back 7 years ago when I started learning Python. In 2017, having worked in many projects, what I have to tell you is this: I never found such case where Python 3 wasn't supported, and I should've learned Py3 to begin with. YMMV, but the fear was unfounded back in 2010, and even more so today.

There are very few projects without Py3 support, and most you'll find without Py3 support is because the project has been dead for quite some time.