template: add the obsfate template Reuse _getobsoletefate and nextvisiblesuccessors to easily get the obsolescence fate of any node. The limitation is that obsfate is as precise as these two functions, which is not great for nextvisiblesuccessors.

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