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.

syntax: re
/figures/[^/]+\.png$
^html/
\.pyc$
~$
\.swp$
\.orig$
\.rej$
\.err$
^tests/easy_run.sh$
^build/
^dist/
^MANIFEST$
^docs/tutorials/.*\.rst$
\.ico$
tests/\.testtimes