In the third case, wihich consider the best approach, do I need to create a new package and go through the review process?
The main package python-astropy can keep providing the python3-astropy subpackage as per the current packagin guidelines (some time in the futere python-astropy will point to the python3 package and not the python2 version with the help of the %pythonprovides macro)
A new package python2-astropy should be added to the distribution