Roadmap to retire pyregion

People should be using https://github.com/astropy/regions but there are blockers:

* https://github.com/astropy/regions/issues/167

We need a roadmap to do the following:

* Port remaining features from pyregion to regions.
* Release regions with these features.
* Decide on a deprecation timeline for the whole pyregion package.
* Communicate the timeline to Astropy community.
* Implement deprecation in pyregion.
* Do one last release of pyregion.
* Archive this repository.

Start by reviewing the blocker in astropy/regions#167 and the listed migration, release, deprecation, communication, and archival steps. No repository files or tests are named; done means an agreed roadmap covering those steps and a decision on the pyregion deprecation timeline.

Written by the indexing model from the issue text.