heptapod-ci: add a basic file to be able to run tests with heptapod Having this yaml file somewhere in the main mercurial repository makes it trivial for contributors using heptapod to run CI on their in-progress work. There are alot of different combination (python2/python3 pure/cext/rust/pypy) to be tested and making sure all of them are covered manually is cumbersome. Automatic CI runnig on draft really helps in that matters. We start small bu later changesets will add more step testing more of the variants. The series is targetted on stable to make it available to the widest amount of contribution possible. The definition of the docker files used for this are available here: https://dev.heptapod.net/octobus/ci-dockerfiles Differential Revision: https://phab.mercurial-scm.org/D8011