obsolete: introduce a _succs class
It will be useful later when we will be adding markers to _succs in order to
represent a successorset with the list of markers from the root to each
successors sets. This information will be needed for the obsfate template I will
introduce.
Makes it a subclass of list so all callers will continue to work.
Run Mercurial tests with Vagrant:
$ vagrant up
$ vagrant ssh -c ./run-tests.sh