Roadmap to retire pyregion

[pllim]

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.

[astrofrog]

Actually for HEALPIX <-> HEALPIX it's also not trivial if you change coordinate system since the pixels are no longer aligned.