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.

At this point in time, if you want to use a library and it isn't compatible with py3, then you probably shouldn't use that library. Essentially all actively maintained libraries are converted, and we are even starting to see libs that do not support py2.

Nowadays it's more likely that a Python 2 user wants to use a library and finds it doesn't work on Python 2, because the developers never targeted nor tested on 2.