MNT: Move nisext imports from distutils to setuptools

[penguinpee]

• TEST: Only use stable argsorts in PARREC tests
• ENH: Warn on invalid MINC2 spacing declarations, treat as missing
• Fix tests for Python 3.12

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (134d469) 92.17% compared to head (65379bb) 92.17%.

Additional details and impacted files

@@             Coverage Diff              @@
##           maint/5.1.x    #1241   +/-   ##
============================================
  Coverage        92.17%   92.17%           
============================================
  Files               98       98           
  Lines            12376    12376           
  Branches          2544     2544           
============================================
  Hits             11407    11407           
  Misses             646      646           
  Partials           323      323           

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[effigies]

I'm okay with this, assuming you wouldn't prefer just to remove nisext from your package altogether.