Python3 is a more streamlined/modern version of Python. Python 2.7.x is the end of the Python2 line. Not all Python2 code will work with Python3, so Python 2.7 will be maintained for those folks who have a fair amount of Python2 code.
Some of the popular Python3 improvements have been ported down to Python 2.7
Sooner or later Python3 will take over, right now there is a somewhat awkward interim time.