Remove numpy hack in io.fits when Numpy minversion is 1.12

[pllim]

I stumbled upon io/fits/_numpy_hacks.py, which has a comment that says it can be retired when numpy/numpy#6430 is merged. That Numpy fix was incorporated into Numpy 1.12, which means we can remove this hack when our minversion for Numpy is 1.12.

(I am tempted to set this to novice but I probably shouldn't.)

[bsipocz]

Given #7058, I'm milestoning this 3.1 to not to forget about it.

[astrofrog]

The best approach would probably be to modify the source code to explicitly have NUMPY_LT_112 so that we can then search/replace whenever we update the minimum Numpy version.

[astrofrog]

This is not 'upstream fix required' since the fix has already been made, right?

[pllim]

Oh, I guess you don't need that label... I just wanted something to indicate that this depends on upstream changes but I guess it is unnecessary given the milestone.

[pllim]

modify the source code to explicitly have NUMPY_LT_112

Seems unnecessary now, as @mhvk has removed it as part of #7058 🎉