refactor: extract obs fate algorithm from _getobsoletereason Refactor _getobsoletereason to extract the obs fate computation into a separate function, it will be used later and put it in obshistory as it will be needed in other files. Rename _getobsoletereason to _getobsoletefateandsuccessors.

# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)