I think nowadays you should just start with Python 3. The majority of the ecosystem is compatible with Python 3, now.

As soon as you have mastered the basics you can learn on how to write Python-3 code that is compatible with Python 2.7. There are some excellent tutorials for that on the net. For new projects there is no need to start with Py27 and then port things to Py3, which is more difficult than writing Py27 compatible Python-3-code.

If you have a mac or linux system, I would just learn whatever is already installed on the machine. For most systems this would be python 2.7.X. This will be a good version to learn the basics of python and most third party libraries will be compatible with it.