RF: Centralize import of decorators from numpy.testing

[yarikoptic]

I kept receiving messages that imports from numpy.testing.decorators are
deprecated (since 1.5 IIRC), so I found that some places still import them
in the test files instead of centraly from nibabel.testing which already
provides imports adaptor. So I RFed to avoid such imports, and moved a
stub for @slow there too

[coveralls]

Coverage increased (+0.0007%) to 91.917% when pulling 0c36551 on yarikoptic:enh-deprec-numpy.testing.decorators into ad0b13f on nipy:master.

[effigies]

This looks reasonable. I'll try to look at the test failures...

[effigies]

We have a minimum version of 1.7.1. I don't think this is guard is necessary anymore.

[effigies]

Oh, the test failure is just the usual AppVeyor nonsense. I thought there was a problem with a recent version of matplotlib. Might be thinking of another project.

[codecov-io]

Codecov Report

Merging #738 into master will decrease coverage by 3.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #738      +/-   ##
==========================================
- Coverage   89.39%   86.24%   -3.15%     
==========================================
  Files          93       93              
  Lines       11476    11474       -2     
  Branches     1992     1992              
==========================================
- Hits        10259     9896     -363     
- Misses        881     1252     +371     
+ Partials      336      326      -10

Impacted Files	Coverage Δ
nibabel/testing/__init__.py	96.8% <100%> (+2.01%)	⬆️
nibabel/dft.py	15.85% <0%> (-65.25%)	⬇️
nibabel/freesurfer/io.py	51.54% <0%> (-42.74%)	⬇️
nibabel/pydicom_compat.py	65% <0%> (-35%)	⬇️
nibabel/environment.py	75% <0%> (-20%)	⬇️
nibabel/pkg_info.py	65.51% <0%> (-13.8%)	⬇️
nibabel/casting.py	81.35% <0%> (-5.94%)	⬇️
nibabel/openers.py	78.22% <0%> (-3.23%)	⬇️
nibabel/__init__.py	91.48% <0%> (-2.13%)	⬇️
nibabel/volumeutils.py	91.78% <0%> (-0.98%)	⬇️
... and 6 more

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[effigies]

Something's weird with the testing because this is never getting hit, even on the numpy==1.7.1 build. See 2007.5, here:

https://codecov.io/gh/nipy/nibabel/commit/0c36551c7398dcfb132fe4ce37b115c23d183b72/build

I know it's not really relevant to this PR, but any thoughts? I checked the travis log, and see no indication that anything other than 1.7.1 was installed.

[yarikoptic]

No immediate idea, I am away from the laptop ATM, but if I were with it I would remove try/except and see how it fails

[effigies]

Ah, it's because numpy.testing.dec has existed forever. It's just an alias of an imported module, and the module it's aliasing has changed from numpy.testing.decorators to numpy.testing._private.decorators.

How about we just drop the try/except bit, and load dec, skipif, and slow?