Remove support for ASTROPY_LT_13 in tests

[cdeil]

This PR removes support for ASTROPY_LT_13 from some tests.

I don't know what Astropy version we support here, but I think it's clear that Astropy < 1.3 is not our target.

@astrofrog @keflavich - OK?

[astrofrog]

I think supporting Astropy 2.x would be fine. Maybe we should make that explicit in the list of dependencies.

[bsipocz]

Is there a roadmap of when to merge this into core, wasn't it 3.2 being aimed for a couple of months ago? If that's still the aim for this package, then we there isn't much need to keep a wide range of supported astropy versions.

[cdeil]

No, there is no roadmap or concrete plan yet. I think astropy.healpix should be done first, because it's the basis for MOC region (#219), and astropy.healpix still needs a lot of work.

I agree requiring Astropy >=2 for astropy.regions would be OK. Probably that's already what we test in CI anyways and all that's needed is a mention in setup.py and install instructions. On the other hand, that adds little value, and most things might actually work with Astropy >= 1.3, so I won't add that now. But if someone thinks it's useful, please send a PR.