MAINT: ignoring IERS warnings

[bsipocz]

Workaround to pass the old python tests: there are no updates for these, so the workaround is to ignore these warnings while we support python 3.8. Alternatively, we could ignore these warnings by default as I don't think they ever have any actual effect on pyvo functionality.

This should fix the python 3.8 job failure

[msdemlei]

Stale IERS data indeed is irrelevant for pyVO; I'd hope one of these days we'll do some smart stuff with TIMESYS or, gasp, proper STC annotation, but by this day I'm afraid python 3.8 will feel like the venerable 1.54 feels to us now.

Ah, except we still have related warnings. The failures we get in some tests from ERFA look a lot like what it would do if it has not got up-to-date IERS data. So, I'm not merging and would suggest to ignore these warnings, too.

[bsipocz]

I'm adding the ErfaWarning ignore in, too.

FWIW, I'm totally open for dropping 3.8 for pyvo1.7, a lot of libraries in fact are already on 3.11+. But this PR will be still handy if we have a 1.6.1 bugfix release before that.

[bsipocz]

The remaining failures should be addressed by #636, so I go ahead and merge this now.