What Python version can you please recommend for a long-term (years) project? Should one use 2.6+ or 3.x is already stable? (only standard libraries are required)

UPDATE: according to the answers below, Python 3.x still has critical bugs. Please also see Python's list of bugs.

This is why you should use Python 3.x:

Python 2.x:

>>>True = False
>>>True
False

Python 3.x:

>>> True = FalseFile "<stdin>", line 1
SyntaxError: assignment to keyword

Source: Strangest language feature

Prejudice: But so many packages are not Python 3 ready yet

This is (a) not true (source) and (b) not important to a beginner.