Deprecation and archiving of this package/repo

[keflavich]

cc @astrofrog @leejjoon

At the astropy coordination meeting, we discussed the status of the pyregion package. I think it is not being maintained any longer, and all of our effort has moved over to astropy regions.

This should be the last PR merged on pyregion. After that, we should close all the issues & PRs and archive the repository.

TODO

• Coordinate with other maintainers.
• Update docs (README and RTD) with banner indicating that package is deprecated. (This PR)
• If RTD has multiple versions, make sure only the one with deprecated message is active. Check the RTD admin panel.
• Close all open issues and PRs as "won't fix".
• Does it need one last release on PyPI?
• Archive this GitHub repository.
• Mark PyPI page (https://pypi.org/project/pyregion/) as archived. PyPI project admin has to do this one.