Replace deprecated/removed FancyURLOpener calls in mig.shared.url module

Our use of `FancyURLOpener` in `mig.shared.url` needs adjustment now that upstream removed it as explained in
https://docs.python.org/3.14/whatsnew/3.14.html#id12

Example failure in recent CI runs like
https://github.com/ucphhpc/migrid-sync/actions/runs/18338864551/job/52229176174