TST: rank removed in Numpy 1.18

[pllim]

Attempt to fix this failure in numpy dev job caused by numpy/numpy#14039

AttributeError: module 'numpy' has no attribute 'rank'

Example log: https://travis-ci.org/astropy/astropy/jobs/566097785

Note for reviewer: Numpy dev job will still fail due to #8976. I am also not sure what to milestone for Numpy 1.18 support.

[bsipocz]

We still have a numpy dev build for on both release branches. So even though I said before that it makes no sense to backport this as the numpy 1.18 release will be after we already release the next LTS, our life might be easier if we just backport.
Alternatively the dev testing jobs could probably be removed from those branches.

@mhvk - what do you think?

[mhvk]

I think that since numpy >=1.17 is for python3 only, and the python2 compatibility stuff is really meant to be removed for 1.18 (IIRC), it makes little sense to test numpy-dev with LTS. I cannot really imagine why anyone would run astropy 2.0 with numpy >=1.17...

[mhvk]

This looks good except sadly I coded it such that you also need to do it in the tests... From the travis log,

astropy/units/tests/test_quantity_non_ufuncs.py:1759: in <module>
945
    np.asscalar, np.rank

[pllim]

@mhvk , like this?

[bsipocz]

I think yes, given he said to do the same for the test. With this assumption I go ahead and merge this.

[pllim]

Just a note that numpy dev will still fail but now it should fail with the 3 tests related to #8976 and not np.rank.

https://travis-ci.org/astropy/astropy/jobs/567131469

[bsipocz]

Thanks @pllim!