Revert mininum numpy change

[mhvk]

This is to ensure astropy LTS does not have to pin. Added an extra test for numpy 1.20 since that was problematic before.

[mhvk]

@avalentino, @astrofrog - I changed the minimum version back to 1.17, given @pllim's comment that it was a problem for LTS (and that really one should not change it in a bug-fix release!). I also added a test with numpy 1.20, which had problems in #101. Here, it seems to pass (perhaps a different ubuntu version and therefore different python? I noticed that on recent python I can no longer pip install numpy==1.17 as the compilation fails...).

Anyway, would be great if you can have a look.

Once merged, all we need to do for a release is to push a tag of 2.0.0.3, right?

[pllim]

Thanks! FWIW, LTS needs numpy>=1.18.

[mhvk]

Thanks, both! I'll merge now...

p.s. Indeed, LTS needs numpy >=1.18, but we used to have 1.17, so I thought I might as well put it back to what it was. With the next ERFA release, we can make a bigger jump.

[pllim]

Thanks again for the quick fix! Related follow-up PRs:

• TST: Relax pyerfa pin for oldestdeps, skip URL in linkcheck (v5.0.x only) astropy/astropy#14567 (LTS)
• TST: Ensure we use minversion of pyerfa in oldestdeps, skip URL in linkcheck astropy/astropy#14568 (main and 5.2.x)