Updates for astropy 6.

[schlafly]

This changes some references to astropy.coordinates.ICRS to astropy.coordinates.ICRS() to reflect some astropy API changes.

It also loosens the tolerance to 40" from 30" for the match to some precomputed transformations. I don't understand why those transformations should have changed by 10", so maybe I should worry about this, but the old tolerance was 30" and the new tolerance is 40", so it's not obviously a big difference.

[schlafly]

This is intended to resolve #157 .

[schlafly]

@dkirkby , would you mind glancing at this PR? I suspect it's fine, but I needed to loosen the tolerance of the comparisons against JPL horizons to 40" from 30". Since I'm not doing any new math and that's not a lot, it doesn't really bother me. On the other hand, I might have expected the tolerances to be closer to 0.1" than 30" and so I can't decide if I should be more bothered.

[coveralls]

coverage: 30.413%. remained the same
when pulling 3463e31 on astropy-6
into fa29a9b on main.

[sbailey]

I'm going to merge this and tag so that we have a working desisurvey + astropy/6.x for the 24.4 software release with Jura.

I agree with @schlafly 's surprise about 40" precision vs. 0.1" precision on these calculations though. Re-pinging @dkirkby for potential followup.

[weaverba137]

I wonder if the previous tests were using a freeze_iers() calculation, whereas in Astropy 6, freeze_iers() is no longer necessary, since the required data files would be downloaded automatically as a separate Python package.