The Most Important Reason

Before getting into the main technical reasons I would like to discuss the one most important social reason for why you should not use Python 3 as a beginner:

THERE IS A HIGH PROBABILITY THAT PYTHON 3 IS SUCH A FAILURE IT WILL KILL PYTHON. Python 3's adoption is really only at about 30% whenever there is an attempt to measure it. Nobody is all that interested in finding out what the real complete adoption is, despite there being fairly simple ways to gather metrics on the adoption. In my honest opinion the language is dead and will not advance far simply because of the technical reasons I list below. These reasons are simple usability problems that can be fixed easily, but the Python project has adopted an arrogant stance that these defects are in fact "features" that are good for you. This is a death sentence for the language, and if you're planning on using Python 3 in the future you will most likely be in trouble.

It's as simple as that. If you learn Python 2, then you can still work with all the legacy Python 2 code in existence until Python dies or you (hopefully) move on. But if you learn Python 3 then your future is very uncertain. You could really be learning a dead language and end up having to learn Python 2 anyway.

Also, it's been over a decade, maybe even multiple decades, and Python 3 still isn't above about 30% in adoption. Even among the sciences where Python 3 is touted as a "success" it's still only around 25-30% adoption. After that long it's time to admit defeat and come up with a new plan.