TEST: Convert nibabel.nicom to unittest/pytest

[effigies]

No description provided.

[codecov]

Codecov Report

Merging #884 into pytest will decrease coverage by 3.9%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           pytest     #884      +/-   ##
==========================================
- Coverage   91.36%   87.45%   -3.91%     
==========================================
  Files          98       98              
  Lines       12166    12169       +3     
  Branches     2186     2186              
==========================================
- Hits        11115    10643     -472     
- Misses        706     1178     +472     
- Partials      345      348       +3

Impacted Files	Coverage Δ
nibabel/pydicom_compat.py	72% <60%> (-28%)	⬇️
nibabel/dft.py	19.93% <0%> (-61.05%)	⬇️
nibabel/freesurfer/io.py	94.22% <0%> (ø)	⬆️
nibabel/cmdline/parrec2nii.py	90.41% <0%> (ø)	⬆️
nibabel/environment.py	95% <0%> (ø)	⬆️
nibabel/conftest.py	100% <0%> (ø)	⬆️
nibabel/casting.py	86.14% <0%> (ø)	⬆️
nibabel/openers.py	94.89% <0%> (ø)	⬆️
nibabel/nicom/csareader.py	81.48% <0%> (-5.93%)	⬇️
... and 38 more

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