3) package the LTS version for python 2 and the new version for python 3.


+1 to this one. something similar will happen with django that is dropping python2 support

https://fedoraproject.org/wiki/Changes/Django20

 
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