Should you only learn Python 3?

It depends. Learning Python 3 will  future-proof your code, and you will use up-to-date features from the developers. However, note that some third-party modules and frameworks lack support for Python 3 and will continue to do so for the near future, until they completely port their libraries to Python 3.

Also, note that some network vendors, such as Cisco, provide limited support for Python 3.x, as most of the required features are already covered in Python 2.x releases. For example, the following are the supported Python versions for Cisco devices; you will see that all devices support 2.x, not 3.x:

Source: https://developer.cisco.com/site/python/