MAINT: Import abstract classes from collections.abc for Python 3.8 compatibility

[larsoner]

Fixes imports that cause:

DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working

[coveralls]

Coverage increased (+0.003%) to 91.827% when pulling 8fa9b29 on larsoner:collections into 8b1a1b2 on nipy:master.

[effigies]

Thanks, this looks good. Very small suggestion.

[codecov-io]

Codecov Report

Merging #711 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #711      +/-   ##
==========================================
+ Coverage   88.83%   88.84%   +<.01%     
==========================================
  Files          93       93              
  Lines       11460    11464       +4     
  Branches     1896     1896              
==========================================
+ Hits        10181    10185       +4     
  Misses        937      937              
  Partials      342      342

Impacted Files	Coverage Δ
nibabel/cifti2/cifti2.py	96.38% <100%> (+0.02%)	⬆️

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