Investigate better bors/GitHub integration?

[duckinator]

Follow-up to #234.

My idea is, basically:

1. Have a Cirrus CI task named something like "CI_Success_task", which does nothing but relies on all other CI tasks.
2. Have bors rely on just that task.

This lets us have all the CI-related configuration that's prone to changing over time in one place.

@nbraud: what are your thoughts on this approach?

[nbraud]

3 main thoughts:

1. I already find .cirrus.yml just large enough to be annoying to navigate, so it's actually slightly more convenient when editing the set of tasks Bors waits on to have it in bors.toml.
2. If we use an alias in each task definition (to captures all instances for all versions of Python we test against), then editing that set would be much, much less frequent and error-prone.
3. Reversing the relationship (i.e. having a depended_on_by on each relevant task, rather than a depends_on list in the CI_Success_task) would be even better, but I don't think the Cirrus config language supports that.

👍 from me overall