Using Dependabot to manage doc build and CI versions

As brought up on the bi-weekly community developers meeting, it sounds like Pandas v1.1.0 is breaking doc builds on RTD. One solution to the issues of frequent breakages in doc builds and CI due to upstream updates is having fixed version lists for all of these, which are then incrementally updated as new versions come out. @dopplershift has done a lot of great work in MetPy getting such a workflow set up with Dependabot (https://github.com/Unidata/MetPy/pull/1410) among other CI updates, and this could be adapted for use here in xarray.

We've generally been quite happy with our updated CI configuration with Dependabot over the past couple weeks. The only major issue has been https://github.com/Unidata/MetPy/issues/1424 / https://github.com/dependabot/dependabot-core/issues/2198#issuecomment-649726022, which has required some contributors to have to delete and recreate their forks in order for Dependabot to not auto-submit PRs to the forked repos.

Any thoughts that you had here @dopplershift would be appreciated!

xref https://github.com/pydata/xarray/issues/4287, https://github.com/pydata/xarray/pull/4296

No file, test, or entry point is named. Start by reviewing the linked xarray issue and pull request, then compare the Dependabot workflow in MetPy pull request 1410. Done should mean an agreed, documented approach for managing pinned documentation-build and CI versions without the fork-related problems described.