I'd suggest OP learns Python 3 and try really hard not to look at Python 2 ever, which hopefully shouldn't be too hard in 2017.
> as long as you can write Python 2/3 compatible code
Maintaining a Python 2 and 3 compatible codebase in a PITA. OP wants to learn a new language, presumably to build things with it. But your advice is that they acquire insight of all the mistakes of a language that was released 7 years ago (that's for 2.7, 2.0 was release in 2000) and will reach end-of-life in 2 years. I'd really like to understand why in the world you would want to learn Python 2 and its “warts“, to then discover that all this stuff are just unnecessary pain points that were removed/refactored/cleaned up in Python 3. In my opinion, Python 2 and 3 are two different languages and should be treated like so.
> [not] doing your users a favor, if you go exclusively Python 3
Actually you are. If you start a project today and make it Python 2 + 3, you are forcing your users (most of whom in 2017 would be proud Python 3 users anyway) to cope with a. all the bugs introduced by the compat code you have to write and b. the delayed releases/bugfixes caused by the subtle Python 2/3 discrepancies you'll face one day or the other.
I will make no comment on the IE6 comparison: this isn't about corporate employees or banking apps from the 90's.
I maintain multiple Py 2-3 compatible codebases, both academically and commercially. It's not hard, because I develop in Python 2, while writing Python 3 compatible code.
Python 2 is a beautiful language as powerful as:
import antigravity
print "Hello, world!"
If Python is a different language and should be treated like so, then I don't want to switch to this other language, or have a project support two different languages (I'd switch to Go if having to switch languages). For all intents and purposes: Python 2 works just fine. Google did not need Python 3 when they first released TensorFlow a year back.
> If you start a project today and make it Python 2 + 3
Then you are compatible will the largest amount of users. That's what matters to me: Somebody on a fresh install being able to pip install my library, no matter if it is Python 2.7+ or 3.4+. All other reasons are politics.
> this isn't about corporate employees or banking apps from the 90's.
No, it's about banking apps from the 00's and 10's.
Does not matter which language you start in, as long as you can write Python 2/3 compatible code. Perhaps starting with Python 2 (and the fantastic "LearnPythonTheHardWay") helps with this. You'll just have to unlearn a few Py2 warts like xrange and using print statements with ellipses.
You are not being progressive, nor doing your users a favor, if you go exclusively Python 3. Even if the language owners decided not to be backwards compatible, your code still can be (and, IMO, proudly should).
As you arrive to a point where you want to use Python 3 exclusive language features, you'll be at least advanced in Python enough, to form your own well-informed opinion on the matter. I myself don't feel like library support for Python 3 should be the deciding factor for a switch. That's like switching to exclusively designing for a certain browser version, because most websites render just fine, leaving users stuck on IE6 without accessible content.