Automerge workflow

In some cases a PR is approved and ready to merge, but is still running on CI. Depending on the tests required, the queue load, etc., this can take a while. For cases like these, it would be helpful to flag a PR as ready to merge. That way as soon as CI passes a bot can come in and merge the PR. This is particularly useful if the reviewer that approved is off doing other things.

We have been doing something like this in conda-forge for a little while now using a custom label. RAPIDS has also recently adopted this process. Am curious if this is something we would be interested in doing in Dask.

FWIW here's [the first GitHub action that I found to do this]( https://github.com/marketplace/actions/auto-merge-pull-request ). Though there are several others like it. My guess is anything like this will require some GH token to setup, but I could be wrong about that