FIX: Set CIFTI-2 dimensions in the last three entries of dim

[effigies]

Builds on and supersedes #929.

[codecov]

Codecov Report

Merging #930 into maint/3.1.x will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           maint/3.1.x     #930      +/-   ##
===============================================
- Coverage        91.86%   91.77%   -0.09%     
===============================================
  Files               97       97              
  Lines            12360    12360              
  Branches          2177     2177              
===============================================
- Hits             11354    11344      -10     
- Misses             675      681       +6     
- Partials           331      335       +4     

Impacted Files	Coverage Δ
nibabel/cifti2/cifti2.py	96.67% <100.00%> (ø)
nibabel/environment.py	75.00% <0.00%> (-20.00%)	⬇️
nibabel/casting.py	85.28% <0.00%> (-0.87%)	⬇️
nibabel/dft.py	80.36% <0.00%> (-0.62%)	⬇️
nibabel/nifti1.py	92.12% <0.00%> (-0.31%)	⬇️

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[mgxd]

Looks good, thanks Chris

[effigies]

Thanks for the review.