List of mirror URLs for IERS download

**Transferred from astropy/astropy**

Both of the USNO hosts (maia.usno.navy.mil, toshi.nofs.navy.mil) for retrieving the IERS Bulletin A dataset are flaky. Instead of a primary URL specified by the configuration setting `utils.iers.iers_auto_url` and mirror URL by `utils.iers.iers_auto_url_mirror`, I suggest a single list of URLs.

Here is what I propose for backward compatibility:

* Understand both `iers_auto_url` and `iers_auto_url_mirror` to be either a string or a list of strings.
* Construct the list of URLs to try by concatenating all of the URLs from `iers_auto_url` and `iers_auto_url_mirror` and removing duplicates.
* Deprecate the `iers_auto_url_mirror` configuration setting for removal in a future release.

Here is a possible default URL list:

* http://maia.usno.navy.mil/ser7/finals2000A.all
* http://toshi.nofs.navy.mil/ser7/finals2000A.all
* ftp://cddis.gsfc.nasa.gov/pub/products/iers/finals2000A.all
* https://datacenter.iers.org/data/9/finals2000A.all

Start by locating the configuration settings utils.iers.iers_auto_url and utils.iers.iers_auto_url_mirror, then trace the IERS Bulletin A download path and its tests. Check how string settings, URL retries, duplicate removal, backward compatibility, and deprecation are currently handled; done means all configured URLs are tried reliably and existing string settings still work.

Written by the indexing model from the issue text.